Modified carbon-coated lithium manganese iron phosphate and preparation method thereof

Abstract

The invention provides modified carbon-coated lithium manganese iron phosphate and a preparation method thereof, and belongs to the technical field of battery electrode materials. The cation-doped lithium manganese iron phosphate material is subjected to two times of carbon coating by adopting transition metal monatomic and a nitrogen-containing carbon source, and the mass ratio of the monatomic transition metal-doped nitrogen-carbon-coated lithium manganese iron phosphate material subjected to the second time of carbon coating, the nitrogen-containing carbon source and transition metal salt is limited; and the obtained modified carbon-coated lithium manganese iron phosphate has excellent conductivity, rate capability and cycle performance, and can be applied to a positive electrode material of a lithium ion battery.

Description

Technical Field

[0001] The present invention relates to the technical field of battery electrode materials, and more specifically, to a modified carbon-coated lithium manganese iron phosphate and a preparation method thereof. Background Art

[0002] Olivine structure lithium manganese iron phosphate (LiFe x Mn 1-x PO4, 0<x<1) is a solid solution phosphate system cathode material formed by replacing iron atoms in lithium iron phosphate with manganese atoms. Compared with the current mainstream iron phosphate and ternary cathode materials, lithium manganese iron phosphate has the advantages of high working voltage platform, high thermal stability, low cost, and green environmental protection. It is regarded as a new cathode material to solve the limited energy density of lithium iron phosphate. However, its inherent poor conductivity and low Li + Diffusion coefficient, Mn 3+ The induced Jahn-Teller effect and manganese dissolution during cycling lead to poor electrochemical performance.

[0003] Existing modification strategies for lithium iron manganese phosphate include cation doping, carbon coating, and particle size nano-sizing. The composite of a highly conductive carbon layer and lithium iron manganese phosphate is an effective method to improve rate performance and cycle stability. It optimizes the conductivity of the material and avoids direct contact between the electrolyte and the active material, thereby improving the cycle stability and rate performance of lithium iron manganese phosphate. The traditional carbon coating method for lithium iron manganese phosphate positive electrode materials is to add inorganic carbon materials such as glucose, polyethylene glycol, sucrose, lactose, carbon black, polypropylene, or organic carbon sources and the main raw materials to mix and sinter to obtain carbon-coated materials in order to obtain carbon-coated lithium iron manganese phosphate positive electrode materials with uniform coating, round particles and good electrical properties. However, the main chemical elements of traditional carbon sources are carbon, hydrogen and oxygen. After high-temperature heat treatment, the carbon layer is mainly composed of carbon elements. It does not have some highly electronegative heteroatoms to dope the carbon layer to regulate the electron distribution, so the modification effect is not good, and excessive addition will cause disadvantages such as difficulty in dispersion, local agglomeration, poor performance, and difficulty in post-processing.

[0004] The prior art discloses a solid-phase preparation method for an ion-doped modified secondary carbon-coated lithium iron manganese phosphate positive electrode material, in which trivalent iron is used as an iron source, and the iron site of the lithium iron manganese phosphate is modified by ion doping and combined with secondary carbon coating and solid-phase high-temperature sintering to synthesize the lithium iron manganese phosphate positive electrode material. However, an excessive amount of iron source will form corresponding metal elements, oxides, nitrides, carbides, etc. during the high-temperature sintering process, which will affect the proportion of active substances in the positive electrode material and reduce the conductivity and structural stability of the positive electrode material. In addition, the lithium source, manganese source, iron source, and phosphorus source are directly mixed with a carbon source for carbon coating, which may result in poor rate performance and cycle performance of the material due to uneven coating.

[0005] Therefore, it is of great research significance and application value to develop a modified carbon-coated lithium iron manganese phosphate with excellent electrical conductivity, rate performance, and cycling performance and its preparation method. Summary of the Invention

[0006] The present invention aims to overcome the deficiencies of the prior art and provides a modified carbon-coated lithium iron manganese phosphate and its preparation method. By using two-time transition metal atom doping and nitrogen-carbon coating, excellent electrical conductivity, rate performance, and cycling performance of the lithium iron manganese phosphate material are achieved.

[0007] Therefore, the primary object of the present invention is to provide a preparation method of modified carbon-coated lithium iron manganese phosphate.

[0008] Another object of the present invention is to provide a modified carbon-coated lithium iron manganese phosphate prepared by the above preparation method.

[0009] Another object of the present invention is to provide an application of the above modified carbon-coated lithium iron manganese phosphate.

[0010] To achieve the above objects, the present invention adopts the following technical solutions:

[0011] The present invention protects a preparation method of modified carbon-coated lithium iron manganese phosphate, which includes the following steps:

[0012] S1. Add the cation-doped lithium iron manganese phosphate material into a solution formed by a nitrogen-containing carbon source and a transition metal salt. After freeze-drying, sinter in an inert atmosphere to obtain a single-atom transition metal-doped nitrogen-carbon-coated lithium iron manganese phosphate material;

[0013] S2. Add the single-atom transition metal-doped nitrogen-carbon-coated lithium iron manganese phosphate material obtained in S1 into a solution formed by a nitrogen-containing carbon source and a transition metal salt again. After freeze-drying, sinter in an inert atmosphere to obtain the modified carbon-coated lithium iron manganese phosphate;

[0014] Among them, the mass ratio of the single-atom transition metal-doped nitrogen-carbon-coated lithium iron manganese phosphate material, the nitrogen-containing carbon source, and the transition metal salt in step S2 is 1000-15000:10-100:1.

[0015] The present invention conducts two-time nitrogen doping and carbon coating on the cation-doped lithium iron manganese phosphate material and simultaneously dopes single atoms of transition metals on the carbon coating layer twice. This is not only beneficial to the formation of a single-atom structural carbon layer of transition metal TM-N-C that improves the electrical conductivity and structural stability of the material, thereby promoting lithium ion migration, inhibiting manganese dissolution, and improving cycling stability; but also beneficial to solving the problems of poor rate performance and cycling stability caused by the uneven carbon coating and poor electrical conductivity improvement effect in the conventional carbon source coating technology.

[0016] When performing the first carbon coating in step S1, a transition metal salt is added to form a single-atom structure of transition metal TM-N-C. The metal particles at the active centers are reduced to the atomic scale, and the metal is uniformly distributed in the nitrogen-rich carbonaceous material in the form of single atoms. It is a carbon-coated material with uniformly distributed atomic active sites, easy to expose, high stability, and not prone to agglomeration. The internal Π-electron conjugate structure endows it with strong coordination ability and electrochemical activity. It is conducive to reducing the transfer resistance, promoting charge transfer, accelerating the lithium-ion migration rate, and increasing the affinity of lithium ions in the carbon material, enhancing the adsorption of lithium ions, and having considerable intrinsic activity. It effectively solves the problems of slow kinetic reaction, serious volume expansion, and manganese dissolution in the lithium iron phosphate electrode material, thereby improving the conductivity and electrochemical performance of the material.

[0017] In step S2, a single-atom transition metal and a nitrogen-containing carbon source are used to perform the second coating and doping on the single-atom transition metal-doped nitrogen-carbon-coated lithium iron phosphate material, so that the material contains more TM-N-C structures, giving play to the synergistic effect between the d electrons of the single-atom transition metal and nitrogen atoms, increasing the conjugation degree of Π electrons, enhancing the electronic interaction between the carbon layer and lithium iron phosphate, improving the electron / ion transfer rate at the coating interface, improving the uniformity of the carbon coating layer, and further improving the rate performance of the material.

[0018] Specifically, the preparation method of the cation-doped lithium iron phosphate material includes the following steps:

[0019] A lithium source, a manganese source, an iron source, and a phosphorus source are added with a cation dopant, mixed to obtain a lithium iron phosphate precursor, and the cation-doped lithium iron phosphate material is obtained after high-temperature sintering with nitrogen.

[0020] Preferably, the lithium source includes but is not limited to one or more of lithium carbonate, lithium hydroxide, or lithium dihydrogen phosphate.

[0021] Preferably, the manganese source includes but is not limited to one or more of manganese tetroxide, manganese dioxide, or manganese carbonate.

[0022] Preferably, the iron source includes but is not limited to one or more of ferrous oxalate, elemental iron, or iron(III) oxide.

[0023] Preferably, the phosphorus source includes but is not limited to one or more of ammonium dihydrogen phosphate, diammonium hydrogen phosphate, or potassium dihydrogen phosphate.

[0024] Preferably, the nitrogen-containing carbon source is selected from one or more of dopamine, monocyanamide, dicyandiamide, melamine, cyanuric chloride, thiourea, or urea.

[0025] Preferably, the temperature of the high-temperature sintering with nitrogen is 650-850 °C. More preferably, it is 750 °C.

[0026] Preferably, the time for high-temperature sintering with nitrogen is 6 - 20 h, and more preferably 10 h.

[0027] Preferably, in step S1 and step S2, the transition metal of the transition metal salt is selected from one or more of iron, manganese, cobalt, copper, zinc, nickel, cerium, or lanthanum.

[0028] More preferably, in step S1 and step S2, the transition metal of the transition metal salt is selected from one or two of iron or manganese.

[0029] Preferably, in step S1 and step S2, the inert gas is one or two of nitrogen or argon.

[0030] Preferably, in step S1 and step S2, the heating rate of sintering is 0.5 - 20 °C / min, and more preferably 5 °C / min.

[0031] Preferably, in step S1 and step S2, the sintering temperature is 700 - 900 °C, and more preferably 800 °C.

[0032] Preferably, in step S1 and step S2, the sintering time is 0.5 - 5 h, and more preferably 1 h.

[0033] Preferably, in the solution formed by the nitrogen-containing carbon source and the transition metal salt in step S1, the mass ratio of the nitrogen-containing carbon source to the transition metal salt is 2 - 20:1.

[0034] Preferably, in step S1, the mass ratio of the nitrogen-containing carbon source to the cation-doped lithium iron manganese phosphate material is 1:10 - 150.

[0035] Preferably, in step S2, the mass ratio of the single-atom transition metal-doped nitrogen-carbon-coated lithium iron manganese phosphate material, the nitrogen-containing carbon source, and the transition metal salt is 2000 - 10000:20 - 100:1.

[0036] Preferably, in step S2, the mass ratio of the single-atom transition metal-doped nitrogen-carbon-coated lithium iron manganese phosphate material, the nitrogen-containing carbon source, and the transition metal salt is 2000 - 5000:20 - 100:1.

[0037] Preferably, in step S2, the mass ratio of the single-atom transition metal-doped nitrogen-carbon-coated lithium iron manganese phosphate material, the nitrogen-containing carbon source, and the transition metal salt is 2000:20:1.

[0038] Preferably, in step S2, the solid-liquid ratio of the single-atom transition metal-doped nitrogen-carbon-coated lithium iron manganese phosphate material, the mixture of the nitrogen-containing carbon source and the transition metal salt, and water is 1:20 - 25 g / mL.

[0039] Preferably, the chemical formula of the cation-doped lithium iron manganese phosphate material is LiMn x Fe 1-x APO4, where 0 < x < 1, A is a cation-doping element, and the content of A does not exceed 10,000 ppm.

[0040] Preferably, the stoichiometric ratio of Li:(Mn + Fe):P elements in the cation-doped lithium iron manganese phosphate material is 0.95 - 1.10:1:0.95 - 1.10.

[0041] Preferably, the A is a cation-doping element, including but not limited to one or more of titanium, magnesium, vanadium, nickel, zinc, copper, tungsten, cerium, cobalt, tin, calcium, aluminum, niobium, chromium, molybdenum, or zirconium.

[0042] Preferably, the cation dopant is one or more of the oxides or metal salts of the corresponding doped cation A.

[0043] Preferably, the addition amount of titanium dioxide is 800 ppm.

[0044] Preferably, the addition amount of magnesium oxide is 2,000 ppm.

[0045] The element of A occupying / replacing the Mn or Fe site forms doping modification at the lattice level.

[0046] The present invention protects a modified carbon-coated lithium iron manganese phosphate prepared by the above preparation method.

[0047] The application of the above modified carbon-coated lithium iron manganese phosphate in the preparation of the cathode material of a lithium-ion battery is also within the protection scope of the present invention.

[0048] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0049] The present invention provides a preparation method of modified carbon-coated lithium iron manganese phosphate. By performing two carbon coatings on the cation-doped lithium iron manganese phosphate material with a nitrogen-containing carbon source doped with transition metal single atoms, and defining the mass ratio of the single-atom transition metal-doped nitrogen-carbon-coated lithium iron manganese phosphate material, the nitrogen-containing carbon source, and the transition metal salt in the second carbon coating, the obtained modified carbon-coated lithium iron manganese phosphate has excellent electrical conductivity, rate performance, and cycling performance. Description of the Drawings

[0050] Figure 1 It is the scanning electron microscope image of Example 1, and the scale is 10 μm.

[0051] Figure 2 It is the scanning electron microscope image of Example 1, and the scale is 2 μm. Detailed Embodiments

[0052] In order to more clearly and completely describe the technical solution of the present invention, the following further details the present invention through specific embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention, and various changes can be made within the scope defined by the rights of the present invention.

[0053] Unless otherwise specified, the reagents and materials used in the following examples are all commercially available.

[0054] Test methods for examples and comparative examples:

[0055] (1) Powder resistance:

[0056] Apply an excitation current to the battery powder samples prepared in the examples and comparative examples using a powder resistivity measuring instrument, and measure the voltage output to obtain the surface resistivity of the material and the powder resistance data;

[0057] (2) Electrochemical performance test method:

[0058] Using the positive electrode materials prepared in the examples and comparative examples as the positive electrode active substances, disperse 4 wt% binder polyvinylidene fluoride, 4 wt% conductive agent carbon black, and 92 wt% positive electrode active substances in the dispersant N-methylpyrrolidone to prepare a battery positive electrode material slurry. Subsequently, coat it on the surface of the carbon-coated aluminum foil and vacuum dry it at 100 °C for 12 h. Punch the positive electrode material pole piece to obtain a circular pole piece, weigh it and record the mass to obtain a positive electrode circular piece. Using the prepared positive electrode circular piece as the positive electrode, a lithium metal sheet as the negative electrode, a polyethylene membrane as the separator, and 1M LiPF6 dissolved in a mixed solution of dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), and ethylene carbonate (EC) (DMC: EMC: EC = 1:1:1 Vol%) as the electrolyte, assemble a CR2032 battery in a glove box with an atmosphere of O2 ≤ 0.01 ppm and H2O ≤ 0.01 ppm, and after standing for 24 h, perform electrochemical performance tests under specific working steps. The electrochemical window for testing is set to 2 - 4.5 V, and charge-discharge tests are performed at a charge-discharge rate of 0.1C / 1C (1C = 170 mAh / g) to obtain relevant electrochemical performance data such as the 0.1C first charge specific capacity (mAh / g), 0.1C first discharge efficiency (%), 0.1C first discharge specific capacity (mAh / g), 1C discharge specific capacity (mAh / g), and 1C 100-cycle capacity retention rate (%).

[0059] Preparation method of cation-doped lithium iron phosphate manganese materials in examples and comparative examples:

[0060] Weigh lithium carbonate, manganese tetraoxide, iron phosphate, and lithium dihydrogen phosphate corresponding to the stoichiometric ratio of Li:Mn:Fe:P elements as 1.06:0.7:0.3:1.02 respectively, disperse them in ultrapure water, and add titanium dioxide and magnesium oxide dopants based on the dosage of 800 ppm Ti doping amount and 2000 ppm Mg doping amount for lithium iron manganese phosphate. Then carry out wet grinding and spray drying to obtain the lithium iron manganese phosphate precursor. Subsequently, place the lithium iron manganese phosphate precursor in a nitrogen roller hearth furnace and heat it to 750 °C at a heating rate of 5 °C / min and keep it at a constant temperature for 10 h. After naturally cooling to room temperature, it is ground by airflow and sieved to obtain the cation-doped lithium iron manganese phosphate material.

[0061] Example 1 Preparation method of modified carbon-coated lithium iron manganese phosphate

[0062] S1. The mass ratio of melamine to ferric chloride hexahydrate is 4:1. Disperse it in ultrapure water and continuously stir to form a solution. Subsequently, add the above-mentioned lithium iron manganese phosphate material to the solution according to the mass ratio of melamine to cation-doped lithium iron manganese phosphate material of 1:50, and continuously stir for 1 h. After freeze-drying, obtain the carbon source-coated lithium iron manganese phosphate precursor, and place it in a nitrogen roller hearth furnace and heat it to 800 °C at a heating rate of 5 °C / min and keep it at a constant temperature for 1 h. After naturally cooling to room temperature, obtain the single-atom iron-doped nitrogen-carbon-coated lithium iron manganese phosphate material;

[0063] S2. According to the mass ratio of single-atom iron-doped nitrogen-carbon composite lithium iron manganese phosphate material, dicyandiamide, and ferric chloride hexahydrate of 2000:20:1, disperse it in ultrapure water at a solid-liquid ratio of 1 g:20 mL and continuously stir to form a solution. After freeze-drying, obtain the secondary atom-doped carbon source-coated lithium iron manganese phosphate precursor, and place it in a nitrogen roller hearth furnace and heat it to 800 °C at a heating rate of 5 °C / min and keep it at a constant temperature for 1 h. After naturally cooling to room temperature, obtain the modified carbon-coated lithium iron manganese phosphate.

[0064] The morphology of the modified carbon-coated lithium iron manganese phosphate material prepared in Example 1 is as Figure 1 and Figure 2 shown, Figure 1 It can be seen that it has good uniformity and dispersion, and there is no obvious free carbon layer on the particle surface, which proves the effectiveness of the secondary atom-doped carbon coating modification strategy of the present invention. Figure 2 It can be seen that the material is approximately spherical after coating.

[0065] Example 2 Preparation method of modified carbon-coated lithium iron manganese phosphate

[0066] S1. The mass ratio of dicyandiamide to ferric chloride hexahydrate is 6:1. It is dispersed in ultrapure water and continuously stirred to form a solution. Subsequently, the above-mentioned lithium iron manganese phosphate material doped with cations is added to the solution at a mass ratio of dicyandiamide to the lithium iron manganese phosphate material doped with cations of 1:50, and stirring is continued for 1 h. After freeze-drying, a lithium iron manganese phosphate precursor coated with a carbon source is obtained, and it is placed in a nitrogen roller hearth furnace and heated to 800 °C at a heating rate of 5 °C / min and kept at a constant temperature for 1 h. After naturally cooling to room temperature, a lithium iron manganese phosphate material doped with single-atom iron and coated with nitrogen and carbon is prepared.

[0067] S2. According to the mass ratio of the lithium iron manganese phosphate material doped with single-atom iron and coated with nitrogen and carbon, dicyandiamide, and ferric chloride hexahydrate of 1000:10:1, it is dispersed in ultrapure water at a solid-liquid ratio of 1 g:25 mL and continuously stirred to form a solution. After freeze-drying, a lithium iron manganese phosphate precursor coated with a carbon source doped with secondary atoms is obtained, and it is placed in a nitrogen roller hearth furnace and heated to 800 °C at a heating rate of 5 °C / min and kept at a constant temperature for 1 h. After naturally cooling to room temperature, a modified carbon-coated lithium iron manganese phosphate is prepared.

[0068] Example 3 A preparation method of modified carbon-coated lithium iron manganese phosphate

[0069] S1. The mass ratio of melamine to manganese chloride tetrahydrate is 3.5:1. It is dispersed in ultrapure water and continuously stirred to form a solution. Subsequently, the above-mentioned lithium iron manganese phosphate material doped with cations is added to it at a mass ratio of melamine to the lithium iron manganese phosphate material doped with cations of 1:25, and stirring is continued for 1 h. After freeze-drying, a lithium iron manganese phosphate precursor coated with a carbon source is obtained, and it is placed in a nitrogen roller hearth furnace and heated to 850 °C at a heating rate of 6 °C / min and kept at a constant temperature for 1 h. After naturally cooling to room temperature, a lithium iron manganese phosphate material doped with single-atom manganese and coated with nitrogen and carbon is prepared.

[0070] S2. According to the mass ratio of the lithium iron manganese phosphate material doped with single-atom manganese and coated with nitrogen and carbon, dicyandiamide, and manganese chloride tetrahydrate of 2000:20:1, it is dispersed in ultrapure water at a solid-liquid ratio of 1 g:20 mL and continuously stirred to form a solution. After freeze-drying, a lithium iron manganese phosphate precursor coated with a carbon source doped with secondary atoms is obtained, and it is placed in a nitrogen roller hearth furnace and heated to 850 °C at a heating rate of 5 °C / min and kept at a constant temperature for 1 h. After naturally cooling to room temperature, a modified carbon-coated lithium iron manganese phosphate is prepared.

[0071] Example 4 A preparation method of modified carbon-coated lithium iron manganese phosphate

[0072] The experimental method is the same as that of Example 1, except that in S2, the mass ratio of the lithium iron manganese phosphate material doped with single-atom iron and coated with nitrogen and carbon, dicyandiamide, and ferric chloride hexahydrate is 5000:20:1.

[0073] Example 5 A preparation method of modified carbon-coated lithium iron manganese phosphate

[0074] The experimental method was the same as that of Example 1, except that in S2, the mass ratio of the single-atom iron-doped nitrogen-carbon composite lithium iron manganese phosphate material, dicyandiamide, and ferric chloride hexahydrate was 15000:20:1.

[0075] Example 6 A preparation method of modified carbon-coated lithium iron manganese phosphate

[0076] The experimental method was the same as that of Example 1, except that in S2, the mass ratio of the single-atom iron-doped nitrogen-carbon composite lithium iron manganese phosphate material, dicyandiamide, and ferric chloride hexahydrate was 2000:100:1.

[0077] Example 7 A preparation method of modified carbon-coated lithium iron manganese phosphate

[0078] S1. The mass ratio of melamine to ferric chloride hexahydrate was 4:1. It was dispersed in ultrapure water and continuously stirred to form a solution. Subsequently, the above lithium iron manganese phosphate material was added to the solution according to the mass ratio of melamine to the cation-doped lithium iron manganese phosphate material of 1:50, and stirring was continued for 1 h. After freeze-drying, a carbon-source-coated lithium iron manganese phosphate precursor was obtained, and it was placed in a nitrogen roller hearth furnace and heated to 800 °C at a heating rate of 5 °C / min and held at a constant temperature for 1 h. After naturally cooling to room temperature, a single-atom iron-doped nitrogen-carbon-coated lithium iron manganese phosphate material was prepared;

[0079] S2. According to the quantitative relationship of the mass ratio of the single-atom iron-doped nitrogen-carbon composite lithium iron manganese phosphate material:dicyandiamide:manganese chloride tetrahydrate of 2000:20:1, with a solid-liquid ratio of 1 g:20 mL, it was dispersed in ultrapure water and continuously stirred to form a solution. After freeze-drying, a secondary atom-doped carbon-source-coated lithium iron manganese phosphate precursor was obtained, and it was placed in a nitrogen roller hearth furnace and heated to 800 °C at a heating rate of 5 °C / min and held at a constant temperature for 1 h. After naturally cooling to room temperature, a modified carbon-coated lithium iron manganese phosphate was prepared.

[0080] Comparative Example 1 A preparation method of modified carbon-coated lithium iron manganese phosphate

[0081] The experimental method was the same as that of Example 1, except that no secondary carbon coating was carried out.

[0082] S1. According to the metering relationship of the mass ratio of melamine to ferric chloride hexahydrate of 4:1, it was dispersed in ultrapure water and continuously stirred to form a solution system A. Subsequently, the above lithium iron manganese phosphate material was added to the solution system A according to the mass ratio of melamine to the above lithium iron manganese phosphate material of 1:50, and stirring was continued for 1 h. After freeze-drying, a carbon-source-coated lithium iron manganese phosphate precursor was obtained, and it was placed in a nitrogen roller hearth furnace and heated to 800 °C at a heating rate of 5 °C / min and held at a constant temperature for 1 h. After naturally cooling to room temperature, a modified carbon-coated lithium iron manganese phosphate was prepared.

[0083] Comparative Example 2 Preparation method of modified carbon-coated lithium iron manganese phosphate

[0084] The experimental method was the same as that of Example 1, except that transition metal atom doping was not carried out during the secondary carbon coating process.

[0085] S1. Step S1. was the same as that of Example 1;

[0086] S2. Dicyandiamide was dispersed in ultrapure water and continuously stirred to form solution system B. After freeze-drying, a nitrogen-doped carbon source-coated lithium iron manganese phosphate precursor was obtained, and it was placed in a nitrogen roller hearth furnace and heated to 800 °C at a heating rate of 5 °C / min and held for 1 h. After naturally cooling to room temperature, modified carbon-coated lithium iron manganese phosphate was prepared.

[0087] Comparative Example 3 Preparation method of modified carbon-coated lithium iron manganese phosphate

[0088] The experimental method was the same as that of Example 1, except that an equal designed amount of glucose was used for the primary carbon coating.

[0089] Weighed an equal mass of glucose as in Example 1 and dispersed it in ultrapure water and continuously stirred to form solution system A. Subsequently, the lithium iron manganese phosphate material doped with cations was added to solution system A in a mass ratio of glucose: cation-doped lithium iron manganese phosphate material of 1:50, and stirring was continued for 1 h. After freeze-drying, a carbon source-coated lithium iron manganese phosphate precursor was obtained, and it was placed in a nitrogen roller hearth furnace and heated to 800 °C at a heating rate of 5 °C / min and held for 1 h. After naturally cooling to room temperature, carbon-coated lithium iron manganese phosphate material was prepared.

[0090] Comparative Example 4 Preparation method of modified carbon-coated lithium iron manganese phosphate

[0091] The experimental method was the same as that of Example 1, except that in S2, the mass ratio of single-atom iron-doped nitrogen-carbon composite lithium iron manganese phosphate material, dicyandiamide, and ferric chloride hexahydrate was 500:20:1.

[0092] Comparative Example 5 Preparation method of modified carbon-coated lithium iron manganese phosphate

[0093] The experimental method was the same as that of Example 1, except that in S2, the mass ratio of single-atom iron-doped nitrogen-carbon composite lithium iron manganese phosphate material, dicyandiamide, and ferric chloride hexahydrate was 20000:20:1.

[0094] Comparative Example 6 Preparation method of modified carbon-coated lithium iron manganese phosphate

[0095] The experimental method was the same as that of Example 1, except that in S2, the mass ratio of single-atom iron-doped nitrogen-carbon composite lithium iron manganese phosphate material, dicyandiamide, and ferric chloride hexahydrate was 2000:5:1.

[0096] Comparative Example 7 Preparation method of modified carbon-coated lithium iron manganese phosphate

[0097] The experimental method was the same as that of Example 1, except that in S2, the mass ratio of the single-atom iron-doped nitrogen-carbon composite lithium iron manganese phosphate material, dicyandiamide, and ferric chloride hexahydrate was 2000:150:1.

[0098] Experimental results:

[0099] The physical and chemical index test data of the modified carbon-coated lithium iron manganese phosphate prepared in the above examples and comparative examples are shown in Table 1:

[0100] Table 1 Material test results of examples and comparative examples

[0101]

[0102]

[0103] It can be seen from Examples 1 to 7 that when the mass ratio of the single-atom transition metal-doped nitrogen-carbon-coated lithium iron manganese phosphate material, nitrogen-containing carbon source, and transition metal salt in step S2 is in the range of 1000-15000:10-100:1, the prepared modified carbon-coated lithium iron manganese phosphate has excellent conductivity, discharge capacity, rate performance, and cycle stability.

[0104] In Comparative Example 1, lithium iron manganese phosphate was subjected to a first carbon coating on the surface with a transition metal salt and a nitrogen-containing carbon source, and no secondary nitrogen-carbon coating was performed, resulting in poor electrical conductivity and structural stability of the material. Although the modified carbon-coated lithium iron manganese phosphate prepared in Comparative Example 2 was subjected to a secondary nitrogen-carbon coating modification, no transition metal atom doping was performed during the secondary carbon coating process, and its electrical conductivity and electrochemical performance were inferior to those of the material in Example 1, indicating that re-doping with transition metal atoms has a promoting effect on improving the rate performance and cycling performance of the material. For the lithium iron manganese phosphate material modified by a first carbon coating with glucose in Comparative Example 3, both the discharge specific capacity and the retention rate of the cycling capacity decreased significantly. In Comparative Example 4, since the mass ratio of the single-atom iron-doped nitrogen-carbon composite lithium iron manganese phosphate material, dicyandiamide, and ferric chloride hexahydrate added in S2 was 500:20:1, that is, the masses of dicyandiamide and ferric chloride hexahydrate added in step S2 were much larger than those in Example 1, and the content of the single-atom iron-doped nitrogen-carbon coated in step S2 was excessive. Although it improved the electrical conductivity of the material to a certain extent, due to the excessive carbon content, some active lithium was embedded in the active sites of the carbon material, reducing the lithium storage capacity. In Comparative Example 5, since the masses of dicyandiamide and ferric chloride hexahydrate added in step S2 were much less than those in Example 1, the content of the single-atom iron-doped nitrogen-carbon coated in step S2 was small, and the synergistic optimization effect of the double-layer single-atom iron-doped nitrogen-carbon could not be efficiently exerted, and the coating property of the material was inferior to that in Example 1. In Comparative Example 6, the mass of dicyandiamide added was much less than that in Example 1, and some of the transition metal iron atoms could not be efficiently doped into the carbon material structure to form other iron-based inactive substances, thereby sacrificing the lithium storage capacity of the material. In Comparative Example 7, since the amount of the metal iron source added was small, the single-atom iron doping density in the single-atom iron-doped nitrogen-carbon constructed in step S2 was low, and the synergistic effect between the d electrons of the transition metal single atoms and the nitrogen atoms in the TM-N-C structure could not be effectively exerted, weakening the conjugation degree of the Π electrons, and thus affecting the electron interaction between the carbon layer and the lithium iron manganese phosphate.

[0105] Obviously, the above-mentioned embodiments of the present invention are merely examples for clearly explaining the present invention, and are not intended to limit the embodiments of the present invention. For those of ordinary skill in the art, other different forms of changes or modifications can be made based on the above description. It is not necessary and impossible to list all the embodiments here. Any modifications, equivalent replacements, and improvements made within the spirit and principle of the present invention shall be included in the protection scope of the claims of the present invention.

Claims

1. A preparation method of modified carbon-coated lithium iron manganese phosphate, characterized in that, It includes the following steps: S1. Add the cation-doped lithium iron manganese phosphate material into the solution formed by the nitrogen-containing carbon source and the transition metal salt. After freeze-drying, sinter it in an inert atmosphere to obtain the single-atom transition metal-doped nitrogen-carbon-coated lithium iron manganese phosphate material; S2. Add the single-atom transition metal-doped nitrogen-carbon-coated lithium iron manganese phosphate material obtained in S1 into the solution formed by the nitrogen-containing carbon source and the transition metal salt again. After freeze-drying, sinter it in an inert atmosphere to obtain the modified carbon-coated lithium iron manganese phosphate; Among them, in step S2, the mass ratio of the single-atom transition metal-doped nitrogen-carbon-coated lithium iron manganese phosphate material, the nitrogen-containing carbon source, and the transition metal salt is 1000-15000:10-100:

1.

2. The preparation method according to claim 1, characterized in that, In the solution formed by the nitrogen-containing carbon source and the transition metal salt in step S1, the mass ratio of the nitrogen-containing carbon source and the transition metal salt is 2-20:

1.

3. The preparation method according to claim 1, characterized in that, In step S1, the mass ratio of the nitrogen-containing carbon source and the cation-doped lithium iron manganese phosphate material is 1:10-150.

4. The preparation method according to claim 1, wherein, In step S2, the mass ratio of the single-atom transition metal-doped nitrogen-carbon-coated lithium iron manganese phosphate material, the nitrogen-containing carbon source, and the transition metal salt is 2000-10000:20-100:

1.

5. The preparation method according to claim 1, characterized in that, In step S2, the solid-liquid ratio of the single-atom transition metal-doped nitrogen-carbon-coated lithium iron manganese phosphate material, the mixture of the nitrogen-containing carbon source and the transition metal salt, and water is 1:20-25 g / mL.

6. The preparation method according to claim 1, wherein In steps S1 and S2, the sintering temperature is 700-900 °C.

7. According to the preparation method described in claim 1, characterized in that, The chemical formula of the cation-doped lithium iron manganese phosphate material is LiMn x Fe 1-x APO4, where 0 < x < 1, A is a cation doping element, and the content of A does not exceed 10,000 ppm.

8. The preparation method according to claim 1, characterized in that, In steps S1 and S2, the transition metal of the transition metal salt is selected from one or more of iron, manganese, cobalt, copper, zinc, nickel, cerium, or lanthanum.

9. A modified carbon-coated lithium iron manganese phosphate, characterized in that, Prepared by the preparation method according to any one of claims 1-8.

10. Use of the modified carbon-coated lithium iron manganese phosphate according to claim 9 in the preparation of a cathode material for a lithium-ion battery.

Images