Coated modified lithium manganese iron phosphate material and preparation method thereof

Abstract

The invention relates to a coated and modified lithium manganese iron phosphate material and a preparation method thereof. The coated and modified lithium manganese iron phosphate material comprises an inner core and a coating layer coating the outer surface of the inner core, wherein the material of the inner core is LiMn < 1-x > Fe < x > PO4, and x is equal to 0.2 to 0.5; and the material of the coating layer comprises an ionic conductive phase and an electronic conductive phase. According to the invention, the ionic conductive phase and the electronic conductive phase are adopted to coat and modify the lithium iron manganese phosphate, so that the polarization phenomenon of the lithium iron manganese phosphate material in the charge-discharge process is effectively improved, and the specific discharge capacity, the rate capability and the cycle stability of the battery positive electrode material are improved.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium-ion batteries, and particularly to a coated and modified lithium iron manganese phosphate material and a preparation method thereof. Background Art

[0002] The polyanion-type cathode material LiMPO4 (M = Fe, Mn, Co, Ni) has the advantages of stable structure, high safety, low cost, and environmental friendliness, and is one of the most promising cathode materials for vehicle power lithium-ion batteries. However, its low electronic conductivity, low ionic conductivity, and poor low-temperature performance limit the practical application of such materials. In recent years, by surface coating with electronic conductors, bulk doping, and preparing nanosized materials, the electrochemical performance of some polyanion-type cathode materials has been significantly improved. In particular, the large-scale application of lithium iron phosphate in vehicle power batteries and energy storage fields has been realized.

[0003] In order to further improve the energy density of lithium iron phosphate batteries, lithium iron manganese phosphate (LMFP) materials have emerged. This material combines the advantages of lithium iron phosphate and lithium manganese phosphate, and has the advantages of high working voltage, good low-temperature performance, and high safety, making it one of the most widely studied lithium-ion cathode materials.

[0004] Lithium iron manganese phosphate materials have problems of low intrinsic conductivity and dissolution of transition metals. Some researchers have improved the materials through technical means such as reducing the particle size of the materials, ion doping, and surface coating with different conductive phases. However, the modification is difficult and the energy density will be reduced. A single carbon coating layer cannot effectively prevent the contact between the electrolyte and the material interface and inhibit the dissolution and precipitation of Mn. Therefore, how to improve the conductivity of lithium iron manganese phosphate materials, inhibit the dissolution and precipitation of transition metals, and improve the cycle stability of the materials has become an urgent technical problem to be solved in this field. Summary of the Invention

[0005] Based on this, in view of the above technical problems existing in the prior art, it is necessary to provide a coated and modified lithium iron manganese phosphate material and a preparation method thereof. The present invention uses an ion conductive phase and an electronic conductive phase to coat and modify lithium iron manganese phosphate, effectively improving the polarization phenomenon during the charge and discharge process of the lithium iron manganese phosphate material, and enhancing the discharge specific capacity, rate performance, and cycle stability of the battery cathode material.

[0006] To solve the above technical problems, the technical solution adopted by the present invention is:

[0007] The first aspect of the present invention provides a coated and modified lithium iron manganese phosphate material, which includes a core and a coating layer coated on the outer surface of the core;

[0008] Among them, the material of the core is LiMn 1-xFe x PO4, where x = 0.2 - 0.5;

[0009] The material of the coating layer includes an ion-conductive phase and an electron-conductive phase.

[0010] Preferably, the electron-conductive phase is located outside the ion-conductive phase.

[0011] Preferably, the ion-conductive phase is Li2SiO3, and the mass percentage in the total mass of the coated and modified lithium iron manganese phosphate material is 1% - 2%.

[0012] Preferably, the electron-conductive phase is amorphous carbon, and the mass percentage in the total mass of the coated and modified lithium iron manganese phosphate material is 1.2% - 3%.

[0013] The second aspect of the present invention provides a method for preparing the coated and modified lithium iron manganese phosphate material proposed in the first aspect, including the following steps:

[0014] Step 1: Dissolve a lithium source compound, a phosphorus source compound, a manganese source compound, an iron source compound, and a complexing agent in a solvent;

[0015] Step 2: After stirring the mixed solution obtained in Step 1, react it in a sealed reaction kettle to obtain a powder suspension;

[0016] Step 3: Filter the powder suspension obtained in Step 2, add deionized water for multiple washings, and dry the washed powder to obtain a pure-phase lithium iron manganese phosphate powder;

[0017] Step 4: Ultrasonically disperse the pure-phase lithium iron manganese phosphate powder obtained in Step 3 and tetraethyl orthosilicate in a solvent. After dropping a mixed solution of lithium hydroxide and ammonia water into the above mixed solution, transfer it to a reaction kettle for the first reaction, and then raise the temperature for the second reaction;

[0018] Step 5: Wash the suspension obtained in Step 4 with deionized water for multiple times, dry it, and then perform dry mixing with a carbon source;

[0019] Step 6: Sinter the powder obtained in Step 5 under gas protection to obtain a C / Li2SiO3-coated and modified lithium iron manganese phosphate material.

[0020] Preferably, the ion concentration ratio of Li:P:(Mn + Fe) in Step 1 is (2.5 - 3):1:1.

[0021] Preferably, the molar ratio of the complexing agent to phosphate in Step 1 is 0.05 - 0.5.

[0022] Preferably, the molar ratio of tetraethyl orthosilicate to pure-phase lithium iron manganese phosphate powder in step 4 is 0.01 - 0.035; the molar ratio of tetraethyl orthosilicate to lithium hydroxide is 1:(2 - 4).

[0023] Preferably, the sintering temperature in step 6 is 675°C - 750°C, and the time is 5h - 10h.

[0024] The third aspect of the present invention provides a cathode material for lithium-ion batteries, using the coated and modified lithium iron manganese phosphate material proposed in the first aspect and the preparation method proposed in the second aspect.

[0025] Due to the adoption of the above technical solutions, the present invention has the following advantages compared with the prior art:

[0026] 1. The present invention uses an ion-conducting phase and an electron-conducting phase to coat and modify lithium iron manganese phosphate. There are lithium silicate ion conductor networks and amorphous carbon electron conductor networks in the lithium iron manganese phosphate material particles, alleviating the anisotropy of the material, effectively improving the polarization phenomenon during the charge and discharge process of the lithium iron manganese phosphate material, and enhancing the discharge specific capacity, rate performance, and cycle stability performance of the cathode material of the battery;

[0027] 2. The present invention also provides a preparation method for the modified lithium iron manganese phosphate material. The preparation method is simple to operate, and it can be seen from SEM that the particle size of the prepared powder material is uniform and controllable. Description of the Drawings

[0028] Figure 1 SEM image of the C / Li2SiO3-coated and modified lithium iron manganese phosphate material prepared in Example 1 of the present invention;

[0029] Figure 2 Charge and discharge performance diagram of the C / Li2SiO3-coated and modified lithium iron manganese phosphate material prepared in Example 1 of the present invention at different rates;

[0030] Figure 3 Charge and discharge performance diagram of the C / Li2SiO3-coated and modified lithium iron manganese phosphate material prepared in Example 2 of the present invention at different rates;

[0031] Figure 4 Charge and discharge cycle life diagram of the coated and modified lithium iron manganese phosphate materials prepared in Example 1 and Comparative Example 1 of the present invention at 1C rate. Detailed Embodiments

[0032] To make the above objects, features, and advantages of the present invention more apparent and understandable, the following provides a detailed description of the specific embodiments of the present invention in conjunction with the accompanying drawings. Many specific details are set forth in the following description to facilitate a thorough understanding of the present invention. However, the present invention can be implemented in many other ways different from those described herein, and those skilled in the art can make similar improvements without departing from the spirit of the present invention. Therefore, the present invention is not limited by the specific embodiments disclosed below.

[0033] It should be noted that the terms "first" and "second" are only used for descriptive purposes and cannot be construed as indicating or implying relative importance or implicitly specifying the quantity of the indicated technical features. Thus, the features defined with "first" and "second" may explicitly or implicitly include one or more of the described features. In the description of the present application, "a plurality" means two or more, unless otherwise specifically defined. In the present application, " / " means "or".

[0034] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by those skilled in the technical field to which the present invention belongs. The terms used in the description of the present invention herein are only for the purpose of describing specific embodiments and are not intended to limit the present invention.

[0035] The lithium iron manganese phosphate material has problems of low intrinsic conductivity and dissolution of transition metals. Some researchers have improved the material through technical means such as reducing the particle size of the material, ion doping, and surface coating with different conductive phases. However, the modification is difficult and the energy density will be reduced. A single carbon coating layer cannot effectively prevent the contact between the electrolyte and the material interface and inhibit the dissolution and precipitation of Mn. Therefore, how to improve the conductivity of the lithium iron manganese phosphate material, inhibit the dissolution and precipitation of transition metals, and improve the cycle stability of the material has become a technical problem urgently to be solved in this field.

[0036] To solve the above technical problems, the present invention discloses a lithium-ion cathode material by sequentially coating an ion conductive phase and an electron conductive phase on the surface of lithium iron manganese phosphate. This cathode material can improve the battery capacity and cycle performance of the battery when used in the battery.

[0037] The first aspect of the present invention provides a coated and modified lithium iron manganese phosphate material, and the coated and modified lithium iron manganese phosphate material includes a core and a coating layer coated on the outer surface of the core;

[0038] Among them, the material of the core is LiMn 1-x Fe x PO4, where x = 0.2 - 0.5;

[0039] The material of the coating layer includes an ion conductive phase and an electron conductive phase.

[0040] By coating an ion-conducting phase on the surface of lithium iron manganese phosphate, the ion-conducting phase provides additional lithium-ion transport channels, reduces the diffusion distance of lithium ions inside the material, thereby improving the rate performance of the material; the high ionic conductivity of the ion-conducting phase helps to accelerate the transport of lithium ions and reduce the kinetic limitations of the electrochemical reaction; it can improve the interfacial contact between lithium iron manganese phosphate and the electrolyte, reduce the interfacial impedance, and improve the cycle stability of the battery.

[0041] The electron-conducting phase can significantly improve the electronic conductivity of the material and reduce the electron transport resistance; the uniform electron-conducting phase coating layer can provide a medium for electron transport and improve the electronic conductivity between particles; at the same time, it can prevent particle growth and agglomeration, thereby maintaining the nanostructure of the material and further improving the diffusion efficiency of lithium ions; in addition, it can also act as a protective layer to prevent the active material from directly contacting the electrolyte, reduce side reactions, and improve the high-temperature performance and cycle performance of the material.

[0042] In one embodiment, the electron-conducting phase is located outside the ion-conducting phase.

[0043] In one embodiment, the ion-conducting phase is Li2SiO3, and its mass percentage in the total mass of the coated and modified lithium iron manganese phosphate material is 1% to 2%.

[0044] In one embodiment, the mass percentage of the ion conductivity in the total mass of the coated and modified lithium iron manganese phosphate material can be selected as 1%, 1.1%, 1.2%, 1.3%, 1.4%, 1.5%, 1.6%, 1.7%, 1.8%, 1.9%, 2%, etc. The above-listed numerical ratios are only examples, and the present invention does not make any restrictions on this.

[0045] In one embodiment, the electron-conducting phase is amorphous carbon, and its mass percentage in the total mass of the coated and modified lithium iron manganese phosphate material is 1.2% to 3%.

[0046] In one embodiment, the mass percentage of the electron conductivity in the total mass of the coated and modified lithium iron manganese phosphate material can be selected as 1.2%, 1.3%, 1.4%, 1.5%, 1.6%, 1.7%, 1.8%, 1.9%, 2%, 2.1%, 2.2%, 2.3%, 2.4%, 2.5%, 2.6%, 2.7%, 2.8%, 2.9%, 3%, etc. The above-listed numerical ratios are only examples, and the present invention does not make any restrictions on this.

[0047] The second aspect of the present invention provides a preparation method of the coated and modified lithium iron manganese phosphate material proposed in the first aspect, including the following steps:

[0048] Step 1: Dissolve the lithium source compound, phosphorus source compound, manganese source compound, iron source compound and complexing agent in a solvent;

[0049] Step 2: After stirring the mixture obtained in Step 1, react it in a sealed reactor to obtain a powder suspension.

[0050] Step 3: Filter the powder suspension obtained in Step 2, add deionized water for multiple washings, and dry the washed powder to obtain a pure-phase lithium iron manganese phosphate powder.

[0051] Step 4: Ultrasonically disperse the pure-phase lithium iron manganese phosphate powder obtained in Step 3 and tetraethyl orthosilicate in a solvent. After dropping a mixed solution of lithium hydroxide and ammonia water into the above-mentioned mixture, transfer it to a reactor for the first reaction, and then raise the temperature for the second reaction.

[0052] Step 5: Wash the suspension obtained in Step 4 with deionized water for multiple times, dry it, and then perform dry mixing with a carbon source.

[0053] Step 6: Sinter the powder obtained in Step 5 under gas protection to obtain a C / Li2SiO3-coated and modified lithium iron manganese phosphate material.

[0054] In one embodiment, the lithium source compound in Step 1 is selected from one or more of lithium hydroxide, lithium sulfate, lithium acetate, lithium dihydrogen phosphate, lithium phosphate, and lithium nitrate.

[0055] In one embodiment, the phosphorus source compound in Step 1 is selected from one or more of ammonium dihydrogen phosphate, diammonium hydrogen phosphate, ammonium phosphate, phosphoric acid, lithium dihydrogen phosphate, and lithium phosphate.

[0056] In one embodiment, the manganese source compound in Step 1 is selected from one or more of manganese sulfate, manganese acetate, and manganese chloride.

[0057] In one embodiment, the iron source compound in Step 1 is selected from one or more of ferrous sulfate and ferrous acetate.

[0058] In the present invention, the specific types of the complexing agent and the solvent in Step 1 are not specifically limited and can be selected according to requirements. Specifically, non-limiting examples of the complexing agent include one or more of citric acid, glycine, malic acid, tartaric acid, nitrilotriacetic acid, and maleic acid, etc.; non-limiting examples of the solvent include one or more of deionized water, ethylene glycol, N-methylpyrrolidone (NMP), dimethyl sulfoxide (DMSO), N,N-dimethylformamide (DMF), tetrahydrofuran (THF), alcohol solvents, and ketone solvents, etc.

[0059] In one embodiment, the ionic concentration ratio of Li:P:(Mn + Fe) in Step 1 is (2.5 - 3):1:1.

[0060] In one embodiment, the ion concentration ratio of Li:P:(Mn+Fe) in step 1 can be selected as 2.5:1:1, 2.6:1:1, 2.7:1:1, 2.8:1:1, 2.9:1:1, 3:1:1, etc. The above-listed numerical ratios are only examples, and the present invention does not impose any restrictions thereon.

[0061] In one embodiment, the ratio of the complexing agent to phosphate in step 1 is from 0.05 to 0.5.

[0062] In one embodiment, the ratio of the complexing agent to phosphate in step 1 can be selected as 0.05, 0.1, 0.15, 0.2, 0.25, 0.3, 0.35, 0.4, 0.45, 0.5, etc. The above-listed numerical ratios are only examples, and the present invention does not impose any restrictions thereon.

[0063] In one embodiment, in step 2, the mixed solution obtained in step 1 is stirred for 0.5 h to 1 h.

[0064] In one embodiment, in step 2, the stirring time of the mixed solution obtained in step 1 can be selected as 0.5 h, 0.6 h, 0.7 h, 0.8 h, 0.9 h, 1 h, etc. The above-listed numerical ratios are only examples, and the present invention does not impose any restrictions thereon.

[0065] In one embodiment, the reaction temperature in step 2 is 180°C to 220°C, and the reaction time is 10 h to 16 h.

[0066] In one embodiment, the reaction temperature in step 2 can be selected as 180°C, 190°C, 200°C, 210°C, 220°C, etc.; the reaction time can be selected as 10 h, 11 h, 12 h, 13 h, 14 h, 15 h, 16 h, etc. The above-listed numerical ratios are only examples, and the present invention does not impose any restrictions thereon.

[0067] In one embodiment, the drying temperature in step 3 is 80°C to 105°C, and the drying time is 20 h to 24 h.

[0068] In one embodiment, the drying temperature in step 3 can be selected as 80°C, 85°C, 90°C, 95°C, 100°C, 105°C, etc.; the drying time can be selected as 20 h, 21 h, 22 h, 23 h, 24 h, etc. The above-listed numerical ratios are only examples, and the present invention does not impose any restrictions thereon.

[0069] In the present invention, the specific types of the solvent in step 4 are not specifically limited and can be selected according to requirements. Specifically, non-limiting examples of the solvent include one or more of deionized water, ethylene glycol, N-methylpyrrolidone (NMP), dimethyl sulfoxide (DMSO), N,N-dimethylformamide (DMF), tetrahydrofuran (THF), alcohol solvents, and ketone solvents.

[0070] In one embodiment, the molar ratio of tetraethyl orthosilicate to pure-phase lithium iron manganese phosphate powder in step 4 is 0.01 to 0.035; the molar ratio of tetraethyl orthosilicate to lithium hydroxide is 1:(2 to 4).

[0071] In one embodiment, the molar ratio of tetraethyl orthosilicate to pure-phase lithium iron manganese phosphate powder in step 4 can be selected from 0.01, 0.015, 0.02, 0.025, 0.03, 0.035; the molar ratio of tetraethyl orthosilicate to lithium hydroxide can be selected from 1:2, 1:2.5, 1:3, 1:3.5, 1:4, etc. The numerical ratios listed above are only examples, and the present invention does not impose any restrictions on this.

[0072] In one embodiment, the concentration of ammonia water in step 4 can be selected according to actual needs, and the present invention does not specifically limit the concentration of ammonia water.

[0073] In one embodiment, the temperature of the first reaction in step 4 is 40°C to 50°C, and the reaction time is 4h to 8h.

[0074] In one embodiment, the temperature of the first reaction in step 4 can be selected from 40°C, 42°C, 44°C, 46°C, 48°C, 50°C, etc.; the reaction time can be selected from 4h, 5h, 6h, 7h, 8h, etc. The numerical ratios listed above are only examples, and the present invention does not impose any restrictions on this.

[0075] In one embodiment, the temperature of the second reaction in step 4 is 100°C to 120°C, and the reaction time is 8h to 16h.

[0076] In one embodiment, the temperature of the second reaction in step 4 can be selected from 100°C, 105°C, 110°C, 115°C, 120°C, etc.; the reaction time can be selected from 8h, 9h, 10h, 11h, 12h, 13h, 14h, 15h, 16h, etc. The numerical ratios listed above are only examples, and the present invention does not impose any restrictions on this.

[0077] In one embodiment, the carbon source in step 5 is selected from one or more of glucose, sucrose, starch, citric acid, polyvinyl pyrrolidone, and polypropylene.

[0078] In one embodiment, the gas in step 6 is one of nitrogen, argon, and helium.

[0079] In one embodiment, the sintering temperature in step 6 is 675°C to 750°C, and the time is 5h to 10h.

[0080] In one embodiment, the sintering temperature in step 6 can be selected from 675 °C, 680 °C, 690 °C, 700 °C, 710 °C, 720 °C, 730 °C, 740 °C, 750 °C, etc.; the reaction time can be selected from 5 h, 6 h, 7 h, 8 h, 9 h, 10 h, etc. The numerical ratios listed above are only examples, and the present invention does not impose any restrictions on this.

[0081] The third aspect of the present invention provides a cathode material for a lithium-ion battery, using the coated and modified lithium iron manganese phosphate material proposed in the first aspect and the preparation method proposed in the second aspect.

[0082] Hereinafter, the embodiments of the present invention will be described more specifically through examples and comparative examples. Among them, all examples and comparative examples are prepared by the same process.

[0083] In order to intuitively and comprehensively reflect the advantages of the present invention, all examples and comparative examples are recorded or tested as follows: initial discharge specific capacity at 0.1C, 0.2C, 0.5C, 1C, 2C, 3C, 4C, and capacity retention rate after 100 cycles at 1C.

[0084] It should be noted that the embodiments of the present invention are not limited to these examples.

[0085] The present invention will be described in detail below in conjunction with specific examples.

[0086] Example 1

[0087] In this example, the material chemical formula of the core of the coated and modified lithium iron manganese phosphate material is LiMn 0.6 Fe 0.4 PO4, the ion conductive phase of the coating layer is Li2SiO3, and the mass percentage in the total mass of the coated and modified lithium iron manganese phosphate material is 1%, and the electronic conductive phase is amorphous carbon, and the mass percentage in the total mass of the coated and modified lithium iron manganese phosphate material is 1.6%. The specific preparation method is as follows:

[0088] Step 1: Dissolve lithium hydroxide, phosphoric acid, ferrous sulfate, manganese sulfate and citric acid in a mixed solution of deionized water and ethylene glycol, where the ion concentration ratio is Li:P:(Mn + Fe) = 3:1:1, and the ratio of citric acid to phosphate is between 0.05;

[0089] Step 2: After stirring the mixed solution obtained in step 1 for 1 h, react it at 180 °C for 16 h in a sealed reaction kettle to obtain a powder suspension;

[0090] Step 3: Filter the powder suspension obtained in step 2 and add deionized water for multiple washes, and dry the washed powder at 80 °C for 24 h to obtain a pure-phase powder of lithium iron manganese phosphate;

[0091] Step 4: Ultrasonically disperse the pure-phase lithium iron manganese phosphate powder and tetraethyl orthosilicate in a mixed solution of water and alcohol. After dropping the mixed solution of lithium hydroxide and ammonia water into the above-mentioned mixed solution, transfer it to a reaction kettle and continuously stir at 40 °C for 8 h for reaction, and then raise the temperature to 100 °C for 16 h. The molar ratio of tetraethyl orthosilicate to lithium hydroxide is 1:2, and the molar ratio of tetraethyl orthosilicate to the pure-phase lithium iron manganese phosphate powder is 0.01;

[0092] Step 5: Wash the suspension obtained in Step 4 with deionized water multiple times, dry it, and then dry-mix it with a carbon source;

[0093] Step 6: Sinter the powder obtained in Step 5 under nitrogen protection at 750 °C for 5 h to obtain a C / Li2SiO3-coated and modified lithium iron manganese phosphate material.

[0094] The SEM image of the coated and modified lithium iron manganese phosphate material prepared in this example is as Figure 1 shown, and the particle size of the material is about 150 nm, and the particles are uniform and fine.

[0095] Use the coated and modified lithium iron manganese phosphate material prepared in this example to assemble a button half-cell according to the following method to test the electrical performance.

[0096] Using the organic solvent N-methylpyrrolidone NMP as the solvent, prepare a slurry by mixing according to the mass ratio of lithium iron manganese phosphate LiMn 0.6 Fe 0.4 PO4: conductive agent conductive carbon black SP: binder polyvinylidene fluoride PVDF of 85:8:7. Coat the mixed slurry on the aluminum foil, and the coating surface density is 80 g / m 2 , and dry it in a forced-air drying oven at 75 °C for 24 h. Die-cut the dried electrode sheet, and the size of the electrode sheet is Using a metal lithium sheet with a diameter of as the negative electrode, Celgard 2400 as the separator, and LiPF6 with a lithium salt concentration of 1 mol / L as the electrolyte, where the mass ratio of the solvents EC:EMC:DMC is 1:1:1. Assemble a CR2032 button cell in a glove box filled with argon. Use a battery test system produced by Wuhan Blue Electric Co., Ltd. to perform charge and discharge tests on the button cell, and the test voltage range is 2-4.5 V. 1C = 160 mAh / g.

[0097] The charge and discharge curves of the LiMn 0.6 Fe 0.4 PO4 material coated with C / Li2SiO3 at different rates are as Figure 2 shown. It can be seen from the charge and discharge curves in the figure that the material has good rate performance.

[0098] Example 2

[0099] In this embodiment, the material chemical formula of the core of the coated and modified lithium iron manganese phosphate material is LiMn 0.65 Fe 0.35 PO4. The ionic conductive phase of the coating layer is Li2SiO3, and its mass percentage in the total mass of the coated and modified lithium iron manganese phosphate material is 2%. The electronic conductive phase is amorphous carbon, and its mass percentage in the total mass of the coated and modified lithium iron manganese phosphate material is 1.6%. The specific preparation method is as follows:

[0100] Step 1: Dissolve lithium hydroxide, phosphoric acid, ferrous sulfate, manganese sulfate and citric acid in a mixed solution of deionized water and ethylene glycol, where the ion concentration ratio is Li:P:(Mn + Fe)=3:1:1, and the ratio of citric acid to phosphate radical is between 0.05;

[0101] Step 2: After stirring the mixed solution obtained in Step 1 for 1 h, react it in a sealed reaction kettle at 180 °C for 16 h to obtain a powder suspension;

[0102] Step 3: Filter the powder suspension obtained in Step 2 and add deionized water for washing multiple times. Dry the washed powder at 80 °C for 24 h to obtain a pure-phase lithium iron manganese phosphate powder;

[0103] Step 4: Ultrasonically disperse the pure-phase lithium iron manganese phosphate powder in Step 3 and tetraethyl orthosilicate in a mixed solution of water and alcohol. After dropping the mixed solution of lithium hydroxide and ammonia water into the above-mentioned mixed solution, transfer it to a reaction kettle and continuously stir it at 40 °C for 8 h for reaction, and then raise the temperature to 100 °C for reaction for 16 h. The molar ratio of tetraethyl orthosilicate to lithium hydroxide is 1:2, and the molar ratio of tetraethyl orthosilicate to the pure-phase lithium iron manganese phosphate powder is 0.03;

[0104] Step 5: Wash the suspension obtained in Step 4 with deionized water for multiple times, dry it, and then perform dry mixing with a carbon source;

[0105] Step 6: Sinter the powder obtained in Step 5 under nitrogen protection at 750 °C for 5 h to obtain a C / Li2SiO3-coated and modified lithium iron manganese phosphate material.

[0106] Assemble the coated and modified lithium iron manganese phosphate material prepared in this embodiment into a button half-cell and test its electrical performance. The LiMn 0.6 Fe 0.4 PO4 material coated with C / Li2SiO3 has charge-discharge curves at different rates as Figure 3 shown. It can be seen from the charge-discharge curves in the figure that this material has good rate performance.

[0107] Example 3

[0108] In this embodiment, the material chemical formula of the core of the coated and modified lithium manganese iron phosphate material is LiMn 0.7 Fe 0.3 PO4. The ion conductive phase of the coating layer is Li2SiO3, and its mass percentage in the total mass of the coated and modified lithium manganese iron phosphate material is 2%. The electronic conductive phase is amorphous carbon, and its mass percentage in the total mass of the coated and modified lithium manganese iron phosphate material is 1.6%. The specific preparation method is as follows:

[0109] Step 1: Dissolve lithium hydroxide, phosphoric acid, ferrous sulfate, manganese sulfate and citric acid in a mixed solution of deionized water and ethylene glycol, where the ion concentration ratio is Li:P:(Mn+Fe)=3:1:1, and the ratio of citric acid to phosphate radical is between 0.05;

[0110] Step 2: After stirring the mixed solution obtained in Step 1 for 1 h, react it in a sealed reaction kettle at 180 °C for 16 h to obtain a powder suspension;

[0111] Step 3: Filter the powder suspension obtained in Step 2 and wash it multiple times with deionized water. Dry the washed powder at 80 °C for 24 h to obtain a pure phase powder of lithium manganese iron phosphate;

[0112] Step 4: Ultrasonically disperse the pure phase powder of lithium manganese iron phosphate in Step 3 and tetraethyl orthosilicate in a mixed solution of water and alcohol. After dropping the mixed solution of lithium hydroxide and ammonia water into the above mixed solution, transfer it to a reaction kettle and continuously stir it at 40 °C for 8 h to react, and then raise the temperature to 100 °C to react for 16 h. The molar ratio of tetraethyl orthosilicate to lithium hydroxide is 1:2, and the molar ratio of tetraethyl orthosilicate to the pure phase powder of lithium manganese iron phosphate is 0.03;

[0113] Step 5: Wash the suspension obtained in Step 4 multiple times with deionized water, dry it, and then dry-mix it with a carbon source;

[0114] Step 6: Sinter the powder obtained in Step 5 under nitrogen protection at 750 °C for 5 h to obtain a C / Li2SiO3-coated and modified lithium manganese iron phosphate material.

[0115] Example 4

[0116] In this embodiment, the material chemical formula of the core of the coated and modified lithium manganese iron phosphate material is LiMn 0.75 Fe 0.25 PO4. The ion conductive phase of the coating layer is Li2SiO3, and its mass percentage in the total mass of the coated and modified lithium manganese iron phosphate material is 2%. The electronic conductive phase is amorphous carbon, and its mass percentage in the total mass of the coated and modified lithium manganese iron phosphate material is 1.6%. The specific preparation method is as follows:

[0117] Step 1: Dissolve lithium hydroxide, phosphoric acid, ferrous sulfate, manganese sulfate and citric acid in a mixed solution of deionized water and ethylene glycol, where the ion concentration ratio is Li:P:(Mn+Fe)=3:1:1, and the ratio of citric acid to phosphate is between 0.05;

[0118] Step 2: After stirring the mixed solution obtained in Step 1 for 1 h, react it in a sealed autoclave at 180 °C for 16 h to obtain a powder suspension;

[0119] Step 3: Filter the powder suspension obtained in Step 2 and add deionized water for washing multiple times. Dry the washed powder at 80 °C for 24 h to obtain a pure phase powder of lithium iron manganese phosphate;

[0120] Step 4: Ultrasonically disperse the pure phase powder of lithium iron manganese phosphate in Step 3 and tetraethyl orthosilicate in a mixed solution of water and alcohol. After dropping the mixed solution of lithium hydroxide and ammonia water into the above mixed solution, transfer it to an autoclave and continuously stir it at 40 °C for 8 h for reaction, and then raise the temperature to 100 °C for reaction for 16 h. The molar ratio of tetraethyl orthosilicate to lithium hydroxide is 1:2, and the molar ratio of tetraethyl orthosilicate to the pure phase powder of lithium iron manganese phosphate is 0.03;

[0121] Step 5: Wash the suspension obtained in Step 4 with deionized water for multiple times, dry it, and then dry-mix it with a carbon source;

[0122] Step 6: Sinter the powder obtained in Step 5 under nitrogen protection at 750 °C for 5 h to obtain a C / Li2SiO3-coated and modified lithium iron manganese phosphate material.

[0123] Example 5

[0124] The chemical formula of the material of the core of the coated and modified lithium iron manganese phosphate material in this example is LiMn 0.8 Fe 0.2 PO4, the ion conductive phase of the coating layer is Li2SiO3, and the mass percentage in the total mass of the coated and modified lithium iron manganese phosphate material is 2%, and the electron conductive phase is amorphous carbon, and the mass percentage in the total mass of the coated and modified lithium iron manganese phosphate material is 1.6%. The specific preparation method is as follows:

[0125] Step 1: Dissolve lithium hydroxide, phosphoric acid, ferrous sulfate, manganese sulfate and citric acid in a mixed solution of deionized water and ethylene glycol, where the ion concentration ratio is Li:P:(Mn+Fe)=3:1:1, and the ratio of citric acid to phosphate is between 0.05;

[0126] Step 2: After stirring the mixed solution obtained in Step 1 for 1 h, react it in a sealed autoclave at 180 °C for 16 h to obtain a powder suspension;

[0127] Step 3: Filter the powder suspension obtained in Step 2, add deionized water for multiple washings, and dry the washed powder at 80°C for 24 h to obtain a pure-phase lithium iron manganese phosphate powder;

[0128] Step 4: Ultrasonically disperse the pure-phase lithium iron manganese phosphate powder in Step 3 and tetraethyl orthosilicate in a mixed solution of water and alcohol. After dropping the mixed solution of lithium hydroxide and ammonia water into the above-mentioned mixed solution, transfer it to a reaction kettle and continuously stir at 40°C for 8 h for reaction, and then raise the temperature to 100°C for reaction for 16 h. The molar ratio of tetraethyl orthosilicate to lithium hydroxide is 1:2, and the molar ratio of tetraethyl orthosilicate to the pure-phase lithium iron manganese phosphate powder is 0.03;

[0129] Step 5: Wash the suspension obtained in Step 4 with deionized water for multiple times, dry it, and then dry-mix it with a carbon source;

[0130] Step 6: Sinter the powder obtained in Step 5 under nitrogen protection at 750°C for 5 h to obtain a C / Li2SiO3-coated and modified lithium iron manganese phosphate material.

[0131] Example 6

[0132] This example provides a C / Li2SiO3-coated and modified lithium iron manganese phosphate material and its preparation method, which are the same as the materials and preparation methods used in Example 1. The difference from Example 1 is only that the mass percentage of Li2SiO3 in the total mass of the C / Li2SiO3-coated and modified lithium iron manganese phosphate material is 1.2%.

[0133] Example 7

[0134] This example provides a C / Li2SiO3-coated and modified lithium iron manganese phosphate material and its preparation method, which are the same as the materials and preparation methods used in Example 1. The difference from Example 1 is only that the mass percentage of Li2SiO3 in the total mass of the C / Li2SiO3-coated and modified lithium iron manganese phosphate material is 1.5%.

[0135] Example 8

[0136] This example provides a C / Li2SiO3-coated and modified lithium iron manganese phosphate material and its preparation method, which are the same as the materials and preparation methods used in Example 1. The difference from Example 1 is only that the mass percentage of Li2SiO3 in the total mass of the C / Li2SiO3-coated and modified lithium iron manganese phosphate material is 2%.

[0137] Comparative Example 1

[0138] This comparative example provides a lithium iron manganese phosphate material coated and modified with C / Li2SiO3 and its preparation method, which are the same as the materials and preparation methods used in Example 1. The difference from Example 1 is only that: Li2SiO3 material is not coated, that is, the mass percentage of Li2SiO3 in the total mass of the coated and modified lithium iron manganese phosphate material is 0%.

[0139] The rate charge-discharge data of the coated and modified lithium iron manganese phosphate materials prepared in Examples 1 to 8 and Comparative Example 1 are shown in Table 1.

[0140] Table 1 Rate charge-discharge data table of the coated and modified lithium iron manganese phosphate materials prepared in Examples 1 to 8 and Comparative Example 1

[0141]

[0142] Among them, in Examples 1, 6, 7, 8 and Comparative Example 1 in the table, only the mass percentage of Li2SiO3 material is changed for comparison. When Li2SiO3 is not coated on the surface of the lithium iron manganese phosphate material, the initial discharge capacity of the battery is significantly lower than the discharge capacity after coating with Li2SiO3 material. As the mass percentage increases, the discharge capacity shows a trend of first increasing and then decreasing, reaching the highest at a mass percentage of 1.5%, and the capacity retention rate at this time exceeds 100%. It shows that coating Li2SiO3 can not only significantly improve the cycle capacity retention rate of the battery, but also maintain stable performance at high rates.

[0143] In the research of lithium iron manganese phosphate cathode materials, it is found through Examples 2 to 5 that changing the ratio relationship between manganese (Mn) and iron (Fe) can significantly affect the electrochemical performance of the materials. The greater the molar ratio of manganese to iron, the lower the discharge capacity and capacity retention rate of the battery. In this application, the material chemical formula of the core of the lithium iron manganese phosphate material is LiMn 0.6 Fe 0.4 PO4, the discharge capacity and capacity retention rate of the battery are both higher, and the comprehensive performance of the battery is more excellent.

[0144] In the present invention, the coating technology can have a significant modification effect on LMFP materials with different ratios, and can effectively improve the cycle capacity retention rate and rate performance of the materials. It mainly utilizes the catalytic property of ammonia water and the induction effect of lithium hydroxide to first form a layer of nano-silica on the surface of lithium iron manganese phosphate, and then, using the silica shell layer as a template, a lithium silicate ion conductive network is formed on the surface of lithium iron manganese phosphate by silica and lithium hydroxide in a hydrothermal environment. Finally, carbon coating is used to improve the electronic conductivity of the material, which can effectively improve the polarization phenomenon during the charge-discharge process of the lithium iron manganese phosphate material, and improve the discharge specific capacity, rate performance and cycle stability performance of the battery cathode material.

[0145] The technical features of the above-described embodiments can be combined arbitrarily. For the sake of brevity of description, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, it should be considered as the scope recorded in this specification.

[0146] The above-described embodiments merely represent several implementation manners of the present invention. The description is relatively specific and detailed, but it should not be construed as a limitation on the scope of the invention patent. It should be noted that for those of ordinary skill in the art, without departing from the concept of the present invention, several modifications and improvements can still be made, and these all belong to the protection scope of the present invention. Therefore, the protection scope of the present invention patent shall be subject to the appended claims.

Claims

1. A coated modified lithium manganese iron phosphate material, characterized in that: The coated modified lithium manganese iron phosphate material comprises a core and a coating layer coated on the outer surface of the core; Wherein, the material of the core is LiMn 1-x Fe x PO4, x = 0.2-0.5; The material of the coating layer includes an ion conductive phase and an electron conductive phase.

2. The coated modified lithium manganese iron phosphate material according to claim 1, characterized in that: The electronically conductive phase is located outside the ionically conductive phase.

3. The coated modified lithium manganese iron phosphate material according to claim 1, characterized in that: The ion conductive phase is Li2SiO3, and its mass percentage in the total mass of the coated modified lithium manganese iron phosphate material is 1% to 2%.

4. According to the coated modified lithium manganese iron phosphate material of claim 1, the electronic conductive phase is amorphous carbon, and its mass percentage in the total mass of the coated modified lithium manganese iron phosphate material is 1.2% to 3%.

5. A method for preparing the coated modified lithium manganese iron phosphate material according to any one of claims 1 to 4, characterized in that: The steps include: Step 1, dissolving a lithium source compound, a phosphorus source compound, a manganese source compound, an iron source compound and a complexing agent in a solvent; Step 2, stirring the mixed solution obtained in step 1, and reacting it in a sealed reactor to obtain a powder suspension; Step 3, filtering the powder suspension obtained in step 2 and adding deionized water to wash it multiple times, and drying the washed powder to obtain pure phase lithium manganese iron phosphate powder; Step 4, ultrasonically dispersing the pure phase powder of lithium manganese iron phosphate and tetraethyl orthosilicate obtained in step 3 in a solvent, dropping a mixed solution of lithium hydroxide and ammonia water into the mixed solution, transferring the mixed solution to a reactor for a first reaction, and then heating the mixture for a second reaction; Step 5, washing the suspension obtained in step 4 with deionized water for multiple times, drying, and then dry-mixing with a carbon source; Step 6: Sinter the powder obtained in step 5 under gas protection to obtain C / Li2SiO3 coated modified lithium manganese iron phosphate material.

6. The method for preparing the coated modified lithium manganese iron phosphate material according to claim 5, characterized in that: The ion concentration ratio of Li:P:(Mn+Fe) in step 1 is (2.5-3):1:

1.

7. The method for preparing the coated modified lithium manganese iron phosphate material according to claim 5, characterized in that: The molar ratio of the complexing agent to the phosphate radical in step 1 is 0.05 to 0.

5.

8. The method for preparing the coated modified lithium manganese iron phosphate material according to claim 5, characterized in that: The molar ratio of tetraethyl orthosilicate to lithium iron manganese phosphate pure phase powder in step 4 is 0.01-0.035; the molar ratio of tetraethyl orthosilicate to lithium hydroxide is 1:(2-4).

9. The method for preparing the coated modified lithium manganese iron phosphate material according to claim 5, characterized in that: The sintering temperature in step 6 is 675° C. to 750° C., and the sintering time is 5 h to 10 h.

10. A positive electrode material for a lithium ion battery, characterized in that: It comprises the coated modified lithium iron manganese phosphate material as described in any one of claims 1 to 4, or the coated modified lithium iron manganese phosphate material prepared by the preparation method of the coated modified lithium iron manganese phosphate material as described in any one of claims 5 to 9.

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