Modified lithium manganese iron phosphate material as well as preparation method and application thereof

Abstract

The invention belongs to the technical field of lithium ion batteries, and particularly relates to a modified lithium iron manganese phosphate material as well as a preparation method and application thereof. The preparation method comprises the following steps: dissolving a hydrophilic carbon source and malic acid in a first solvent to obtain a solution A; dissolving a manganese source, an iron source, a lithium source and a phosphorus source in a second solvent to obtain a solution C, and carrying out hydrothermal treatment on the solution C to obtain a solution B; mixing the solution A and the solution B, and carrying out ultrasonic spray atomization treatment, drying treatment and first heat treatment to obtain a lithium ferric manganese phosphate core material; and mixing the lithium iron manganese phosphate core material, a surfactant and titanium dioxide, and carrying out second heat treatment on the obtained mixture to obtain the modified lithium iron manganese phosphate material. The lithium manganese iron phosphate precursor solution is prepared by a hydrothermal method, and then the precursor solution and the solution containing the hydrophilic carbon source and the malic acid are subjected to ultrasonic spray pyrolysis treatment, so that the obtained modified lithium manganese iron phosphate material is good in processability and has remarkably improved cycle performance.

Description

Technical Field

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

[0002] With the growth of energy demand and the increasing severity of environmental problems, as an important energy storage device, the research and development of the cathode material of lithium-ion batteries have received extensive attention. Lithium iron manganese phosphate (LMFP) is regarded as an upgraded version of lithium iron phosphate. The two have the same theoretical capacity, but lithium iron manganese phosphate has a higher voltage platform compared with lithium iron phosphate, and the energy density is about 20% higher than that of lithium iron phosphate. It is expected to break through the upper limit of the current battery energy density and is a promising new cathode material for the next generation.

[0003] However, when preparing the lithium iron manganese phosphate cathode material, the conventionally used high-temperature solid-phase sintering method will form residual lithium and residual alkali on the surface of the lithium iron manganese phosphate cathode material, which will cause the slurry to gel during the pulping process of the cathode material, resulting in uneven coating surface density, reducing the processing performance, and easily causing the phenomenon of lithium precipitation due to uneven lithium distribution and the side reaction between the residual alkali and the electrolyte, thereby affecting the electrochemical performance of the battery, which greatly restricts the large-scale application of the lithium iron manganese phosphate cathode material.

[0004] Currently, the main improvement technical solutions include water washing, surface coating, etc. However, water washing requires a large amount of water resources and has problems such as lithium loss and material structure damage, resulting in a decrease in the material capacity. Although a large amount of research work shows that introducing a coating layer on the surface of the cathode material can avoid direct contact between the cathode material and the electrolyte, reduce the probability of side reactions, and effectively improve the structural stability of the material, thereby improving the electrochemical performance of the material, but the existence of most coating layers will reduce the overall conductivity of the cathode material and is not conducive to the migration of lithium ions, resulting in the difficulty of fully exerting the discharge capacity and cycle capacity performance of the material, and the improvement effect is limited. Summary of the Invention

[0005] The purpose of the present invention is to provide a modified lithium iron manganese phosphate material, a preparation method thereof, and an application thereof for the problems that the existing lithium iron manganese phosphate material has residual alkali and high internal resistance, resulting in poor processing performance of the cathode material and poor cycle performance of the lithium-ion battery.

[0006] In the first aspect, the present invention provides a preparation method of a modified lithium iron manganese phosphate material.

[0007] Specifically, the preparation method of the modified lithium iron manganese phosphate material comprises the following steps: S1. Dissolve a hydrophilic carbon source and malic acid in a first solvent to obtain solution A; dissolve a manganese source, an iron source, a lithium source, and a phosphorus source in a second solvent, and after hydrothermal treatment of the obtained solution C, obtain solution B containing a lithium iron manganese phosphate precursor; mix solution A and solution B, and after ultrasonic spray atomization treatment and drying treatment, perform a first heat treatment on the obtained product to obtain a lithium iron manganese phosphate core material, wherein the chemical general formula of the lithium iron manganese phosphate core material is LiMn x Fe 1-x PO4 / C, 0 < x < 1; S2. Mix the lithium iron manganese phosphate core material obtained in step S1, a surfactant, and titanium dioxide, and perform a second heat treatment on the obtained mixture to form a lithium titanium compound coating layer on the surface of the lithium iron manganese phosphate core material, thereby obtaining the modified lithium iron manganese phosphate material.

[0008] In a preferred embodiment, in step S1, the molar ratio of the iron source in solution C to malic acid in solution A is 1: (1 - 3).

[0009] In a preferred embodiment, in step S1, the dosage of the hydrophilic carbon source is 5 - 15% of the total mass of the added manganese source, iron source, lithium source, and phosphorus source.

[0010] In a preferred embodiment, in step S1, the mass ratio of solution A to solution B during mixing is (0.1 - 0.3):1.

[0011] In a preferred embodiment, the chemical general formula of the lithium iron manganese phosphate core material is LiMn x Fe 1-x PO4 / C, wherein 0.5 ≤ x ≤ 0.8.

[0012] In a preferred embodiment, the hydrophilic carbon source is selected from at least one of chitosan, starch, gum arabic, and bone glue.

[0013] In a preferred embodiment, the first solvent is an acidic solution or water.

[0014] In a preferred embodiment, the manganese source is selected from at least one of manganese hydroxide, manganese nitrate, manganese acetate, manganese sulfate, and manganese carbonate.

[0015] In a preferred embodiment, the iron source is selected from at least one of iron nitrate, iron oxalate, iron sulfate, and iron phosphate.

[0016] In a preferred embodiment, the lithium source is selected from one of lithium hydroxide, lithium nitrate, lithium acetate, lithium carbonate, and lithium oxalate.

[0017] In a preferred embodiment, the phosphorus source is selected from at least one of lithium dihydrogen phosphate, sodium phosphate, ammonium dihydrogen phosphate, diammonium hydrogen phosphate, potassium phosphate, and phosphoric acid.

[0018] In a preferred embodiment, the second solvent is water.

[0019] In a preferred embodiment, in step S1, the conditions of the hydrothermal treatment include: a temperature of 160 - 200 °C and a time of 6 - 10 h.

[0020] In a preferred embodiment, in step S1, the frequency of the ultrasonic spray atomization treatment is 1 - 3 MHz.

[0021] In a preferred embodiment, in step S1, the temperature of the drying treatment is 300 - 400 °C.

[0022] In a preferred embodiment, in step S1, the conditions of the first heat treatment include: an inert gas atmosphere, a temperature of 500 - 600 °C, and a holding time of 1 - 3 h.

[0023] In a preferred embodiment, in step S2, the mass ratio of the lithium iron manganese phosphate core material, the surfactant, and titanium dioxide is 100:(1 - 4):(0.5 - 1.5).

[0024] In a preferred embodiment, the surfactant is a non - ionic surfactant.

[0025] In a preferred embodiment, the lithium - titanium compound coating layer contains lithium titanate and / or titanium dioxide.

[0026] In a preferred embodiment, in step S2, the second heat treatment is carried out successively at a first plateau temperature and a second plateau temperature, and the first plateau temperature is less than the second plateau temperature.

[0027] In a preferred embodiment, in step S2, the first plateau temperature is 300 - 400 °C and the holding time is 6 - 10 h.

[0028] In a preferred embodiment, in step S2, the second plateau temperature is 600 - 700 °C and the holding time is 10 - 15 h.

[0029] Second, the present invention provides a modified lithium iron manganese phosphate material prepared by the above - mentioned method.

[0030] Third, the present invention provides the application of the above - mentioned modified lithium iron manganese phosphate material in a lithium - ion battery.

[0031] Beneficial effects: In the preparation method provided by the present invention, the lithium iron manganese phosphate precursor solution is first prepared by a hydrothermal method, and then the precursor solution is subjected to ultrasonic spray pyrolysis treatment with a solution containing a hydrophilic carbon source and malic acid. On the one hand, it is beneficial to uniformly dope the carbon material into the interior and surface of the lithium iron manganese phosphate during the synthesis of the lithium iron manganese phosphate core material, reduce the internal resistance of the material, and improve the conductivity. On the other hand, it can reduce the residual alkali content of the material and improve the processing performance, so as to better exert the electrochemical performance of the material. Then, the obtained lithium iron manganese phosphate core material is coated and modified with titanium dioxide in the presence of a surfactant, which is beneficial to further improve the electronic conductivity and ionic conductivity of the material, as well as improve the structural stability of the material and reduce side reactions. Therefore, the obtained modified lithium iron manganese phosphate material has significantly improved cycle performance. Description of the Drawings

[0032] Figure 1 It is a viscosity change diagram of the lithium iron manganese phosphate materials obtained in the examples and comparative examples. Detailed Embodiments

[0033] The preparation method of the modified lithium iron manganese phosphate material provided by the present invention includes the following steps:

[0034] S1. Dissolve a hydrophilic carbon source and malic acid in a first solvent to obtain solution A; dissolve a manganese source, an iron source, a lithium source, and a phosphorus source in a second solvent, and after hydrothermal treatment of the obtained solution C, obtain solution B containing a lithium iron manganese phosphate precursor; mix solution A and solution B, and after ultrasonic spray atomization treatment and drying treatment, the obtained product is subjected to a first heat treatment to obtain a lithium iron manganese phosphate core material, where the chemical general formula of the lithium iron manganese phosphate core material is LiMn x Fe 1-x PO4 / C, 0 < x < 1, x can be 0.1, 0.2, 0.3, 0.4, 0.5, 0.8, 0.9 or any value between them. Preferably, 0.5 ≤ x ≤ 0.8;

[0035] S2. Mix the lithium iron manganese phosphate core material obtained in step S1, a surfactant, and titanium dioxide, and the obtained mixture is subjected to a second heat treatment to form a lithium titanium compound coating layer on the surface of the lithium iron manganese phosphate core material to obtain a modified lithium iron manganese phosphate material.

[0036] In the present invention, in step S1, the molar ratio of the iron source in solution C to malic acid in solution A is preferably 1:(1 - 3), such as 1:1, 1:1.5, 1:2, 1:2.5, 1:3 or any value between them.

[0037] In the present invention, in step S1, the mass ratio of solution A to solution B during mixing is preferably (0.1 - 0.3):1, such as 0.1:1, 0.15:1, 0.2:1, 0.25:1, 0.3:1 or any value therebetween.

[0038] In the present invention, the hydrophilic carbon source is a kind of carbon material precursor substance containing at least one of hydrophilic groups such as -OH, -CHO, -COOH, -NH2, etc., which can form good interaction with water molecules and thus exhibit high solubility or wettability in water. Specific examples of the hydrophilic carbon source include but are not limited to at least one of chitosan, starch, gum arabic, and bone glue. The dosage of the hydrophilic carbon source is preferably 5 - 15% of the total mass of the added manganese source, iron source, lithium source, and phosphorus source, such as 5%, 8%, 10%, 12%, 15% or any value therebetween.

[0039] In the present invention, the first solvent is selected to be able to dissolve the hydrophilic carbon source, preferably an acidic solution or water. The acidic substance in the acidic solution is not specifically limited as long as it can promote the dissolution of the hydrophilic carbon source in water, and is preferably at least one of acetic acid, formic acid, acetic acid, and phosphoric acid.

[0040] In the present invention, the manganese source is a kind of reagent commonly used in the existing methods for preparing lithium iron manganese phosphate materials, and the present invention does not make special limitations on it. Specifically, it can be selected from at least one of manganese hydroxide, manganese nitrate, manganese acetate, manganese sulfate, and manganese carbonate.

[0041] In the present invention, the iron source is a kind of reagent commonly used in the existing methods for preparing lithium iron manganese phosphate materials, and the present invention does not make special limitations on it. Specifically, it can be selected from at least one of iron nitrate, iron oxalate, iron sulfate, and iron phosphate.

[0042] In the present invention, the lithium source is a kind of reagent commonly used in the existing methods for preparing lithium iron manganese phosphate materials, and the present invention does not make special limitations on it. Specifically, it can be selected from one of lithium hydroxide, lithium nitrate, lithium acetate, lithium carbonate, and lithium oxalate.

[0043] In the present invention, the phosphorus source is a kind of reagent commonly used in the existing methods for preparing lithium iron manganese phosphate materials, and the present invention does not make special limitations on it. Specifically, it can be selected from at least one of lithium dihydrogen phosphate, sodium phosphate, ammonium dihydrogen phosphate, diammonium hydrogen phosphate, potassium phosphate, and phosphoric acid.

[0044] In the present invention, the second solvent is preferably water.

[0045] In the present invention, the method for obtaining Solution A may be to simultaneously add a hydrophilic carbon source and malic acid into a first solvent and dissolve them to obtain Solution A, or to separately dissolve the hydrophilic carbon source and malic acid in the first solvent to obtain Solution A1 and Solution A2, and then mix Solution A1 and Solution A2 to obtain Solution A. It may also be to first dissolve the hydrophilic carbon source or malic acid in the first solution and then add the remaining other component to dissolve to obtain Solution A. Among them, the dissolution method may be mechanical stirring and / or ultrasonic dispersion.

[0046] In a specific embodiment, the process of dissolving the hydrophilic carbon source and malic acid in the first solvent to obtain Solution A is as follows: Add the hydrophilic carbon source to an acidic solution and dissolve it by mechanical stirring, add malic acid to water and dissolve it by mechanical stirring, and then mix the two obtained solutions and perform ultrasonic dispersion and mechanical stirring to obtain Solution A. Among them, the time of mechanical stirring and ultrasonic dispersion is not specifically limited, and it is subject to complete dissolution to obtain a uniform solution. The specific time can be selected according to the actual situation.

[0047] In the present invention, during the process of dissolving the manganese source, iron source, lithium source, and phosphorus source in the second solvent, there is no specific limitation on the order of adding the manganese source, iron source, lithium source, and phosphorus source to the second solvent for dissolution. It may be added simultaneously for dissolution, or several of the raw materials may be first added to the second solvent for dissolution and then the remaining raw materials may be added for dissolution. The specific process may be as follows: Add the manganese source and iron source to a part of the second solvent for dissolution, add the lithium source and phosphorus source to another part of the second solvent for dissolution, and then mix the two obtained solutions to obtain Solution C.

[0048] In the present invention, before the hydrothermal treatment of Solution C, a pH adjustment step is preferably included, specifically as follows: Mix Solution C with an alkaline substance to adjust the pH to 6-7. The alkaline substance includes but is not limited to at least one of sodium hydroxide, potassium hydroxide, sodium carbonate, potassium carbonate, and ammonia monohydrate.

[0049] In the present invention, in step S1, the conditions of the hydrothermal treatment preferably include: the temperature is 160-200 °C, such as 160 °C, 170 °C, 180 °C, 190 °C, 200 °C or any value between them; the time is 6-10 h, such as 6 h, 7 h, 8 h, 9 h, 10 h or any value between them.

[0050] In the present invention, in step S1, the frequency of the ultrasonic spray atomization treatment is preferably 1-3 MHz, such as 1 MHz, 1.5 MHz, 1.8 MHz, 2 MHz, 2.5 MHz, 3 MHz or any value between them.

[0051] In the present invention, in step S1, the temperature of the drying treatment is preferably 300 - 400 °C, such as 300 °C, 320 °C, 350 °C, 380 °C, 400 °C or any value therebetween.

[0052] In the present invention, in step S1, the conditions of the first heat treatment preferably include: an inert gas atmosphere, such as nitrogen, argon, xenon, etc.; a temperature of 500 - 600 °C, such as 500 °C, 520 °C, 550 °C, 580 °C, 600 °C or any value therebetween; and a heat preservation time of 1 - 3 h, such as 1 h, 1.5 h, 2 h, 2.5 h, 3 h or any value therebetween.

[0053] In a specific embodiment, the implementation method of step S1 may be as follows: Add a hydrophilic carbon source to an acidic solution and dissolve it by mechanical stirring. Add malic acid to water and dissolve it by mechanical stirring. Then mix the two obtained solutions and obtain solution A after ultrasonic dispersion and mechanical stirring. Dissolve a manganese source and an iron source in a part of a second solvent, dissolve a lithium source and a phosphorus source in another part of the second solvent, and then mix the two obtained solutions to obtain solution C. Adjust the pH of solution C to 6 - 7 with an alkaline substance and perform hydrothermal treatment at a temperature of 160 - 200 °C for 6 - 10 h to obtain solution B containing a lithium iron manganese phosphate precursor. Mix solution A and solution B and perform atomization treatment with an ultrasonic nebulizer under the condition of a frequency of 1 - 3 MHz. Introduce the atomized aerosol into a vertical quartz device and perform drying treatment at an air flow rate of 5 - 15 L / min (such as 5 L / min, 8 L / min, 10 L / min, 12 L / min, 15 L / min or any value therebetween). After heat-treating the obtained powder material in an inert gas atmosphere and at a temperature of 400 - 600 °C for 1 - 3 h, a lithium iron manganese phosphate core material can be obtained.

[0054] In the present invention, in step S2, the mass ratio of the lithium iron manganese phosphate core material, the surfactant, and titanium dioxide is preferably 100:(1-4):(0.5-1.5). Based on 100 parts by weight of the lithium iron manganese phosphate core material, the amount of the surfactant is preferably 1-4 parts by weight, such as 1, 2, 3, 4 parts by weight or any value therebetween. At this time, it is more conducive to reducing the internal resistance and improving the uniformity of subsequent titanium dioxide coating, thereby improving the cycle performance of the battery. When the amount of the surfactant is excessive, it may have an adverse effect on the capacity performance of the positive electrode material. Based on 100 parts by weight of the lithium iron manganese phosphate core material, the amount of titanium dioxide is preferably 0.5-1.5 parts by weight, such as 0.5, 0.8, 1, 1.2, 1.5 parts by weight or any value therebetween. On the one hand, titanium dioxide can react with the residual alkali on the surface of the lithium iron manganese phosphate core material to reduce the amount of residual alkali on the material surface and improve the slurry processing performance. On the other hand, it can improve the crystal structure of the lithium iron manganese phosphate material, increase the electronic conductivity and ionic conductivity, thereby improving the charge and discharge efficiency of the lithium battery and obtaining better discharge capacity performance and cycle performance. Controlling the amount of titanium dioxide within the above preferred range is more conducive to ensuring the uniform coating of titanium dioxide on the surface of the lithium iron manganese phosphate core material while further optimizing the crystal form of the lithium iron manganese phosphate material, improving the structural stability of the material, promoting the diffusion and insertion / extraction of lithium ions, and thus improving the discharge capacity performance and cycle performance of the obtained modified lithium iron manganese phosphate material. When the amount of titanium dioxide is excessive, it may generate too much lithium-containing titanium compound due to the reaction of titanium dioxide with the residual alkali, which may cause lattice distortion, affect the diffusion and insertion / extraction of lithium ions, and thus reduce the capacity of lithium iron phosphate and deteriorate the cycle stability.

[0055] In the present invention, the function of the surfactant is to promote the uniform coating of titanium dioxide on the surface of the lithium iron manganese phosphate core material and become a carbon material after heat treatment, further improving the conductivity of the material. The surfactant is preferably a non-ionic surfactant. Compared with anionic surfactants and cationic surfactants, the selected non-ionic surfactant at this time has the advantages of being well soluble in acidic, alkaline, and neutral solutions, not easily reacting with metal ions, and being less affected by electrolytes. Specific examples thereof include, but are not limited to, at least one of polyglycol tert-octyl phenyl ether, coconut fatty acid diethanolamide, coconut oil amide propyl amine oxide, triglycerol monostearate, and monolaurin.

[0056] In the present invention, the lithium titanium compound coating layer preferably includes lithium titanate and / or titanium dioxide.

[0057] In the present invention, in step S2, the mixing treatment method of the lithium iron manganese phosphate core material, surfactant and titanium dioxide is preferably ball milling treatment. The rotation speed of the ball milling treatment is preferably 400-600 rpm, such as 400 rpm, 450 rpm, 500 rpm, 550 rpm, 600 rpm or any value therebetween; the time is preferably 2-6 h, such as 2 h, 3 h, 4 h, 5 h, 6 h or any value therebetween.

[0058] In the present invention, in step S2, the second heat treatment can be carried out at a platform temperature or at two different platform temperatures. The second heat treatment is preferably carried out at a first platform temperature and a second platform temperature in sequence, and the first platform temperature is less than the second platform temperature.

[0059] Furthermore, the first platform temperature is preferably 300-400 °C, such as 300 °C, 320 °C, 350 °C, 380 °C, 400 °C or any value therebetween; the holding time is currently 6-10 h, such as 6 h, 7 h, 8 h, 9 h, 10 h or any value therebetween.

[0060] Furthermore, the second platform temperature is preferably 600-700 °C, such as 600 °C, 620 °C, 650 °C, 680 °C, 700 °C or any value therebetween; the holding time is preferably 10-15 h, such as 10 h, 11 h, 12 h, 13 h, 14 h, 15 h or any value therebetween.

[0061] The present invention will be described in detail below through specific examples. The examples are intended to explain the present invention and should not be construed as limiting the present invention. For those not specifying specific techniques or conditions in the examples, the techniques or conditions described in the literature in the field or according to the product specification are followed. For the reagents or instruments not indicating the manufacturer, they are all conventional products that can be obtained through commercial purchase.

[0062] Example 1

[0063] This example is used to illustrate the preparation of a modified lithium iron manganese phosphate material, which is specifically as follows:

[0064] S1. At room temperature, weigh 0.169 mol of chitosan (purchased from Zhejiang Jinke Biochemistry Co., Ltd., CAS No. 9012-76-4, molecular weight 161.16 g / mol) and dissolve it in 1000 mL of acetic acid solution with a concentration of 1 wt%, and continuously stir at 25 °C for 2 h to obtain solution A1; weigh malic acid according to the molar ratio of iron source (iron phosphate) to malic acid of 1:2 and add it to 1000 mL of deionized water, and continuously stir at 25 °C for 2 h to obtain solution A2; mix solution A1 and solution A2 and ultrasonically disperse them in a 100 W ultrasonic cleaner for 1 h, and then stir for 1 h to obtain mixed solution A;

[0065] At room temperature, add 0.5 mol of lithium carbonate (Li2CO3) and 1 mol of phosphoric acid (H3PO4) to 450 mL of deionized water and stir with a magnetic stirrer for 30 min to obtain a lithium phosphate solution; add 0.6 mol of manganese carbonate (MnCO3) and 0.4 mol of iron phosphate (FePO4) to 150 mL of deionized water and stir with a magnetic stirrer for 30 min to obtain a metal salt solution; then mix the lithium phosphate solution and the metal salt solution and adjust the pH to 6.5 with sodium hydroxide solution, and transfer it to a high-pressure reactor, and react at 180 °C for 8 h to obtain solution B;

[0066] Mix solution B and solution A according to a mass ratio of 1:0.15 and atomize them with an ultrasonic atomizer under the condition of a frequency of 1.8 MHz, and then introduce the atomized aerosol into a vertical quartz tube heated to 400 °C for drying treatment, with an air flow rate of 10 L / min. Finally, heat the obtained powder in an air atmosphere at 500 °C for 1 h to obtain the lithium iron manganese phosphate core material;

[0067] S2. Weigh the lithium iron manganese phosphate core material, polyethylene glycol tert-octyl phenyl ether, and titanium dioxide prepared in step S1 according to a mass ratio of 100:3:1.5 respectively, then add the three to a ball mill and ball mill at a speed of 500 rpm for 4 h. Finally, sinter at 350 °C for 8 h in a nitrogen atmosphere, directly raise the temperature to 650 °C and continue sintering for 13 h. After cooling to room temperature, grind thoroughly with a mortar to obtain the modified lithium iron manganese phosphate material.

[0068] Example 2

[0069] This example is used to illustrate the preparation of a modified lithium iron manganese phosphate material, which is specifically as follows:

[0070] Prepare the modified lithium iron manganese phosphate material according to the method of Example 1, except that the mass ratio of the lithium iron manganese phosphate core material to polyethylene glycol tert-octyl phenyl ether in step S2 is 100:1, and the other conditions are the same as those in Example 1, and the modified lithium iron manganese phosphate thus prepared.

[0071] Example 3

[0072] This example is used to illustrate the preparation of a modified lithium iron manganese phosphate material, which is as follows:

[0073] The modified lithium iron manganese phosphate material was prepared according to the method of Example 1, except that the mass ratio of the lithium iron manganese phosphate core material to polyoxyethylene tert-octyl phenyl ether in step S2 was 100:2, and the other conditions were the same as those in Example 1. The modified lithium iron manganese phosphate thus prepared.

[0074] Example 4

[0075] This example is used to illustrate the preparation of a modified lithium iron manganese phosphate material, which is as follows:

[0076] The modified lithium iron manganese phosphate material was prepared according to the method of Example 1, except that the mass ratio of the lithium iron manganese phosphate core material to polyoxyethylene tert-octyl phenyl ether in step S2 was 100:4, and the other conditions were the same as those in Example 1. The modified lithium iron manganese phosphate thus prepared.

[0077] Example 5

[0078] This example is used to illustrate the preparation of a modified lithium iron manganese phosphate material, which is as follows:

[0079] The modified lithium iron manganese phosphate material was prepared according to the method of Example 1, except that the mass ratio of the lithium iron manganese phosphate core material to titanium dioxide in step S2 was 100:0.5, and the other conditions were the same as those in Example 1. The modified lithium iron manganese phosphate thus prepared.

[0080] Example 6

[0081] This example is used to illustrate the preparation of a modified lithium iron manganese phosphate material, which is as follows:

[0082] The modified lithium iron manganese phosphate material was prepared according to the method of Example 1, except that the mass ratio of the lithium iron manganese phosphate core material to titanium dioxide in step S2 was 100:1, and the other conditions were the same as those in Example 1. The modified lithium iron manganese phosphate thus prepared.

[0083] Example 7

[0084] This example is used to illustrate the preparation of a modified lithium iron manganese phosphate material, which is as follows:

[0085] S1. At room temperature, 0.127 mol of starch (purchased from Shanghai MacLean Co., Ltd., CAS No. 9005-84-9, Article No.: S817547, molecular weight: 342.29 g / mol) was weighed and dissolved in 1000 mL of 1 wt% acetic acid solution, and stirred continuously at 25°C for 2 h to obtain solution A1; malic acid was weighed and added to 1000 mL of deionized water in a molar ratio of 1:1 between the iron source (ferric oxalate) and malic acid, and stirred continuously at 25°C for 2 h to obtain solution A2; solution A1 and solution A2 were mixed and ultrasonically dispersed in a 100 W ultrasonic cleaner for 1 h, and then stirred for 1 h to obtain a mixed solution A;

[0086] At room temperature, 1 mol of lithium hydroxide (LiOH) and 1 mol of sodium phosphate (Na3PO4) were added to 450 mL of deionized water and stirred with a magnetic stirrer for 30 min to obtain a lithium phosphate solution; 0.8 mol of manganese acetate (Mn(CH3COO)2) and 0.1 mol of ferric oxalate (Fe2(C2O4)3) were added to 150 mL of deionized water and stirred with a magnetic stirrer for 30 min to obtain a metal salt solution; then the lithium phosphate solution and the metal salt solution were mixed and the pH was adjusted to 6.5 with a sodium hydroxide solution, and then transferred to a high-pressure reactor, and reacted at 160°C for 10 h to obtain a solution B;

[0087] Solution B and solution A were mixed at a mass ratio of 1:0.15 and then atomized using an ultrasonic sprayer at a frequency of 1.8 MHz. The atomized aerosol was then introduced into a vertical quartz chamber heated to 400°C for drying and pyrolysis at an air flow rate of 10 L / min. Finally, the obtained powder was treated at 400°C in an air atmosphere for 3 h to obtain a lithium iron manganese phosphate core material.

[0088] S2. The lithium iron manganese phosphate core material, coconut oil diethanolamide, and titanium dioxide prepared in step S1 were weighed in a mass ratio of 100:3:1.5, and then added into a ball mill, and ball-milled at a speed of 500 rpm for 4 hours. Finally, the mixture was sintered at 300°C in a nitrogen atmosphere for 10 hours, and then directly heated to 600°C and continued to be sintered for 15 hours. After cooling to room temperature, the modified lithium iron manganese phosphate material was obtained by grinding it thoroughly with a mortar.

[0089] Example 8

[0090] This example is used to illustrate the preparation of a modified lithium manganese iron phosphate material, as follows:

[0091] S1. At room temperature, 0.238 mol of chitosan (purchased from Zhejiang Jinke Biochemical Co., Ltd., CAS No. 9012-76-4, molecular weight 161.16 g / mol) was weighed and dissolved in 1000 mL of 1 wt% acetic acid solution, and stirred continuously at 25°C for 2 h to obtain solution A1; malic acid was weighed and added to 1000 mL of deionized water in a molar ratio of iron source (ferric nitrate) to malic acid of 1:3, and stirred continuously at 25°C for 2 h to obtain solution A2; solution A1 and solution A2 were mixed and ultrasonically dispersed in a 100 W ultrasonic cleaner for 1 h, and then stirred for 1 h to obtain a mixed solution A;

[0092] At room temperature, 1 mol of lithium nitrate (LiNO3) and 1 mol of ammonium dihydrogen phosphate (NH4H2PO4) were added to 450 mL of deionized water and stirred with a magnetic stirrer for 30 min to obtain a lithium phosphate solution; 0.6 mol of manganese nitrate (Mn(NO3)2) and 0.4 mol of iron nitrate (Fe(NO3)3) were added to 150 mL of deionized water and stirred with a magnetic stirrer for 30 min to obtain a metal salt solution; then the lithium phosphate solution and the metal salt solution were mixed and the pH was adjusted to 6.5 with a sodium hydroxide solution, and then transferred to a high-pressure reactor, and reacted at 200°C for 6 hours to obtain a solution B;

[0093] Solution B and solution A were mixed at a mass ratio of 1:0.15 and then atomized using an ultrasonic sprayer at a frequency of 1.8 MHz. The atomized aerosol was then introduced into a vertical quartz chamber heated to 400°C for drying and pyrolysis at an air flow rate of 10 L / min. Finally, the obtained powder was treated at 600°C in an air atmosphere for 2 h to obtain a lithium iron manganese phosphate core material.

[0094] S2. The lithium iron manganese phosphate core material, triglycerol monostearate and titanium dioxide prepared in step S1 were weighed in a mass ratio of 100:3:1.5, and then added into a ball mill, and ball-milled at 500 rpm for 4 h. Finally, the mixture was sintered at 400 °C in a nitrogen atmosphere for 6 h, and then directly heated to 700 °C and sintered for 10 h. After cooling to room temperature, the modified lithium iron manganese phosphate material was obtained by grinding the mixture in a mortar.

[0095] Comparative Example 1

[0096] The modified lithium manganese iron phosphate material was prepared according to the method of Example 1, except that the same molar amount of n-hexanoic acid was used instead of chitosan in step S1, and the other conditions were the same as those in Example 1, thereby preparing the modified lithium manganese iron phosphate.

[0097] Comparative Example 2

[0098] The modified lithium iron manganese phosphate material was prepared according to the method of Example 1, except that malic acid was not added in step S1, and the other conditions were the same as those in Example 1. The obtained modified lithium iron manganese phosphate was prepared.

[0099] Comparative Example 3

[0100] The reference lithium iron manganese phosphate material was prepared according to the method of Example 1, except that poly(ethylene glycol) tert-octyl phenyl ether was not added in step S2, and the other conditions were the same as those in Example 1. The obtained reference lithium iron manganese phosphate was prepared.

[0101] Comparative Example 4

[0102] The reference lithium iron manganese phosphate material was prepared according to the method of Example 1, except that step S2 was not carried out, and the other conditions were the same as those in Example 1. The obtained reference lithium iron manganese phosphate was prepared.

[0103] Comparative Example 5

[0104] The reference lithium iron manganese phosphate material was prepared according to the method of Comparative Example 4, except that malic acid was not added in step S1, and the other conditions were the same as those in Comparative Example 4. The obtained reference lithium iron manganese phosphate was prepared.

[0105] Comparative Example 6

[0106] This comparative example is used to illustrate the preparation of a reference lithium iron manganese phosphate material, which is specifically as follows:

[0107] S1. The lithium source, phosphorus source, manganese source, and iron source were weighed according to a molar ratio of 1:1:0.6:0.4, and then all the weighed raw materials and chitosan were added in a mass ratio of 1:0.15. Malic acid was weighed and added according to a molar ratio of carbon source to malic acid of 1:2. Then acetic acid and deionized water were added, and ball milling was carried out at a speed of 300 rpm for 7 h. Then spray drying was carried out at 200 °C, and then calcination was carried out at 600 °C for 8 h to obtain a carbon-coated lithium iron manganese phosphate core material;

[0108] S2. It was carried out according to step S2 of Example 1, and the obtained reference lithium iron manganese phosphate material was prepared.

[0109] Test Example

[0110] The lithium iron manganese phosphate prepared in the above examples and comparative examples was used as the cathode material to prepare a button battery and test the relevant performance according to the following method. The obtained results are shown in Table 1.

[0111] 1. Preparation of button battery: Weigh the lithium iron phosphate, PVDF glue, carbon black (SP), and carbon nanotubes (CNT) obtained from the examples and comparative examples according to the mass ratio of 95:2:0.8:0.6 and the condition that the total weight is 10 g. Pour the weighed materials into a ball milling tank and stir with a high-speed swing ball mill for 10 min to obtain a uniformly stirred positive electrode paste. Then evenly coat it on the current collector aluminum foil, dry it in a vacuum drying oven at 105°C for 4 h, and roll it to a compaction density of 2.35 mg / cm 3 After that, perform die-cutting and weighing, and then dry it in a vacuum drying oven at 105°C for 2 h to obtain the positive electrode sheet for standby. Complete the assembly process of the button battery in a glove box filled with argon in the order of negative electrode shell, negative electrode sheet, spring sheet, gasket, lithium sheet, separator, electrolyte, positive electrode sheet, and positive electrode shell. Finally, seal the battery with a battery sealer. Among them, use the lithium sheet as the negative electrode sheet, the separator is a polypropylene film, and the electrolyte is the existing conventional one.

[0112] 2. Performance testing

[0113] (1) Charge-discharge capacity test: At 25°C, after standing the button battery assembled by Method 1 for 12 h, charge it at a constant current of 0.1C until the voltage reaches 4.2V, stand for 5 min, then discharge it at a constant current of 0.1C until 2.5V, stand for 5 min, record the first-cycle discharge capacity and calculate the discharge specific capacity.

[0114] (2) Cycle performance test: At 25°C, charge the button battery assembled by Method 1 at a constant current of 1C until the voltage reaches 4.2V, stand for 5 min, then discharge it at a constant current of 1C until 2.5V, stand for 5 min. Thus, one charge-discharge cycle is completed. Perform 300 charge-discharge cycles at 25°C according to the above conditions, record and calculate the discharge specific capacity after each cycle, and obtain the capacity retention rate by dividing the discharge specific capacity after the 300th cycle by the discharge specific capacity after the first cycle.

[0115] (3) Dielectric sheet resistance test: After measuring the thickness of the rolled positive electrode sheet in Method 1, place it on the four probes of the electrode sheet resistance meter, set the pressure to 300 MPa, the pressure holding time to 5 s, and the temperature to 25°C, and input the measured thickness for testing.

[0116] (4) DCIR Test: Charge the button battery assembled by Method 1 at a constant current of 0.5C until the voltage reaches 4.2V, then discharge it at a constant current of 0.5C until the voltage reaches 2.5V. Conduct 3 complete capacity tests to obtain the average capacity C1. Then, charge it at a constant current of 1 / 2C1 for 60 minutes until it reaches 100% SOC (State of Charge), let it stand for 2 hours, and record the voltage at this time as U1. Discharge it at a constant current of 2C for 10 seconds, and record the voltage after discharge as U2. Calculate the DCIR value using the formula R = (U1 - U2) / I, where I = capacity * rate, and the current is calculated by combining the two formulas of time = capacity / current. Generally, after setting the process steps, the current will be automatically calculated according to the given values.

[0117] (5) Viscosity Test: After filling a 50 mL beaker with the positive electrode slurry in Method 1, use a digital display viscometer to measure the viscosity of the slurry after standing for 0h, 1h, 2h, 3h, 4h, 5h, and 6h respectively. Try to record the value when the included angle is 50% during the measurement.

[0118] Table 1

[0119]

[0120] As can be seen from the results in Table 1, compared with Comparative Examples 1 - 6, the lithium iron manganese phosphate materials provided by Examples 1 - 8 of the present invention have lower discharge DCIR and better cycle capacity retention rate. The viscosity of the slurry changes less after standing for 6 hours, and the processing performance is better.

[0121] Although the embodiments of the present invention have been shown and described above, it can be understood that the above embodiments are exemplary and should not be construed as limiting the present invention. Those of ordinary skill in the art can make changes, modifications, substitutions, and variations to the above embodiments within the scope of the present invention without departing from the principles and purposes of the present invention.

Claims

1. A preparation method of a modified lithium iron manganese phosphate material, characterized in that, The preparation method includes the following steps: S1. Dissolve the hydrophilic carbon source and malic acid in the first solvent to obtain solution A; dissolve the manganese source, iron source, lithium source, and phosphorus source in the second solvent, and after hydrothermal treatment of the obtained solution C, obtain solution B containing the lithium manganese iron phosphate precursor; mix solution A and solution B, and after ultrasonic spray atomization treatment and drying treatment, perform the first heat treatment on the obtained product to obtain the lithium manganese iron phosphate core material; wherein, the chemical general formula of the lithium manganese iron phosphate core material is LiMn x Fe 1-x PO4 / C, 0 < x < 1; S2. Mix the lithium iron manganese phosphate core material, surfactant, and titanium dioxide obtained in step S1, and perform a second heat treatment on the obtained mixture to form a lithium titanate coating layer on the surface of the lithium iron manganese phosphate core material, thereby obtaining a modified lithium manganese iron phosphate material.

2. The preparation method of the modified lithium iron manganese phosphate material according to claim 1, wherein In step S1, the molar ratio of the iron source in solution C to malic acid in solution A is 1:(1 - 3); Preferably, the dosage of the hydrophilic carbon source is 5 - 15% of the total mass of the added manganese source, iron source, lithium source, and phosphorus source; Preferably, the mass ratio of solution A to solution B during mixing is (0.1 - 0.3):1; Preferably, the chemical general formula of the lithium iron manganese phosphate core material is LiMn x Fe 1-x PO4 / C, where 0.5 ≤ x ≤ 0.

8.

3. The preparation method of the modified lithium iron manganese phosphate material according to claim 1, characterized in that, The hydrophilic carbon source is selected from at least one of chitosan, starch, arabic gum, and bone glue; Preferably, the first solvent is an acidic solution or water.

4. The preparation method of the modified lithium iron manganese phosphate material according to claim 1, characterized in that, The manganese source is selected from at least one of manganese hydroxide, manganese nitrate, manganese acetate, manganese sulfate, and manganese carbonate; Preferably, the iron source is selected from at least one of iron nitrate, iron oxalate, iron sulfate, and iron phosphate; Preferably, the lithium source is selected from one of lithium hydroxide, lithium nitrate, lithium acetate, lithium carbonate, and lithium oxalate; Preferably, the phosphorus source is selected from at least one of lithium dihydrogen phosphate, sodium phosphate, ammonium dihydrogen phosphate, diammonium hydrogen phosphate, potassium phosphate, and phosphoric acid; Preferably, the second solvent is water.

5. The preparation method of the modified lithium iron manganese phosphate material according to claim 1, characterized in that, In step S1, the conditions for the hydrothermal treatment include: a temperature of 160 - 200 °C and a time of 6 - 10 h; Preferably, the frequency of the ultrasonic spray atomization treatment is 1 - 3 MHz; Preferably, the temperature for the drying treatment is 300 - 400 °C; Preferably, the conditions for the first heat treatment include: an inert gas atmosphere, a temperature of 500 - 600 °C, and a heat preservation time of 1 - 3 h.

6. The preparation method of the modified lithium iron manganese phosphate material according to claim 1, wherein In step S2, the mass ratio of the lithium iron manganese phosphate core material to the surfactant and titanium dioxide is 100:(1 - 4):(0.5 - 1.5).

7. The preparation method of the modified lithium iron manganese phosphate material according to claim 1, characterized in that, The surfactant is a non-ionic surfactant; Preferably, the lithium titanate coating layer contains lithium titanate and / or titanium dioxide.

8. The preparation method of the modified lithium iron manganese phosphate material according to claim 1, wherein In step S2, the second heat treatment is carried out successively at a first platform temperature and a second platform temperature, and the first platform temperature is less than the second platform temperature; Preferably, the first platform temperature is 300 - 400 °C and the heat preservation time is 6 - 10 h; Preferably, the second platform temperature is 600 - 700 °C and the heat preservation time is 10 - 15 h.

9. A modified lithium manganese iron phosphate material prepared by the method according to any one of claims 1 - 8.

10. Application of the modified lithium manganese iron phosphate material according to claim 9 in a lithium ion battery.

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