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

Abstract

The application provides a preparation method of a carbon-coated lithium manganese iron phosphate positive electrode material, which comprises the following steps: preparing a precursor containing lithium, manganese, iron and phosphorus; performing surface modification on the precursor to adhere ionic polymers to the surface of the precursor to obtain an ionic precursor; mixing the ionic precursor with a carbon source; and then calcining the mixture under a protective atmosphere, wherein the calcination system is heated to 630-670 DEG C and kept for 3-5 hours, and the carbon-coated lithium manganese iron phosphate positive electrode material is prepared. The ionic polymers are closely adhered to the surface of the precursor by surface modification of the precursor, and the carbon source is uniformly adhered to the surface of the ionic precursor by using the characteristics of high dispersity and large loading area of the ionic polymers, so that the carbon-coated layer has excellent film forming properties, is dense and complete, is uniformly dispersed and is not agglomerated, and the conductivity and electrochemical properties of the lithium manganese iron phosphate positive electrode material are improved.

Description

Technical Field

[0001] The present invention belongs to the technical field of lithium-ion batteries, and in particular relates to a carbon-coated lithium manganese iron phosphate positive electrode material and a preparation method thereof. Background Art

[0002] Olivine-type phosphate cathode material LiFe x Mn 1-x PO4(0 <x<1)体积比能量和质量比能量相对较高,可大电流充放电、开路电压高且环境友好无污染、安全性好、价格低廉等优点而受到广泛关注。然而橄榄石结构的磷酸盐类正极材料其本身电子导电性极差,严重阻碍了大规模的推广以及应用,要实现磷酸盐正极材料推向商业化的大批量生产和应用,综合考虑提高磷酸盐类正极材料的电子电导率成为研究的当务之急。常见的改性方法是通过向磷酸锰铁锂正极材料LiFe x Mn 1-x PO4(0 <x<1)中加入葡萄糖或蔗糖等碳源,通过高温烧结在LiFe x Mn 1-x PO4(0 <x<1)正极材料表面包覆一层碳,以此来提高材料的导电性。但是该方法存在一定的缺陷,葡萄糖、蔗糖等碳源无法均匀的分散在正极材料的表面,无法在正极材料表面形成均匀的碳包覆层,由此合成的LiFe x Mn 1-x PO4(0 <x<1)正极材料具有较低的电子电导率和循环性能。

[0003] Based on this, how to achieve uniform coating of a carbon layer on the surface of the lithium manganese iron phosphate positive electrode material to improve the conductivity of the positive electrode material is a technical problem that technicians in this field urgently need to solve in their research. Summary of the Invention

[0004] The object of the present invention is to provide a method for preparing a carbon-coated lithium manganese iron phosphate positive electrode material. The preparation method can make the carbon coating layer have excellent film-forming properties, be dense and complete, be evenly dispersed, and not agglomerate, effectively avoid contact between the lithium manganese iron phosphate positive electrode material and the electrolyte, inhibit the dissolution of the Mn element, and thereby improve the conductivity and electrochemical properties of the lithium manganese iron phosphate positive electrode material. In addition, the preparation method is simple to operate, low in cost, and suitable for industrial production.

[0005] According to one aspect of the present invention, a method for preparing a carbon-coated lithium manganese iron phosphate positive electrode material is provided, comprising the following steps: prefabricating a precursor containing lithium, manganese, iron and phosphorus elements, surface-modifying the precursor to attach an ionic polymer to the surface of the precursor to obtain an ionic precursor, mixing the ionic precursor with a carbon source material, and then calcining the resulting mixture under a protective atmosphere. During the calcination process, the calcination system is heated to 630-670°C and maintained at this temperature for 3-5 hours to obtain a carbon-coated lithium manganese iron phosphate positive electrode material. The present invention modifies the surface of the precursor so that the ionic polymer is tightly attached to the surface of the precursor. The ionic polymer has the characteristics of high dispersion and large loading area, so that the subsequently added carbon source is evenly attached to the surface of the ionic precursor. The carbon coating layer thus formed has excellent film-forming properties, is dense and complete, uniformly dispersed, and does not agglomerate. It effectively avoids contact between the lithium manganese iron phosphate positive electrode material and the electrolyte, inhibits the dissolution of the Mn element, and thus improves the conductivity and electrochemical properties of the lithium manganese iron phosphate positive electrode material. The preparation method provided by the present invention can synthesize the carbon-coated lithium manganese iron phosphate positive electrode material with low energy consumption and in a short time, is simple to operate, low cost, and suitable for industrial production.

[0006] Preferably, in the ionic precursor, the ionic polymer is bonded to the surface of the precursor through chemical bonds.

[0007] Preferably, in the ionic precursor, the ionic polymer is bonded to the surface of the precursor via an ester bond. The present invention uses an esterification reaction to uniformly attach the ionic polymer to the surface of the precursor via chemical ester bonds, and the reaction conditions are mild and the operation is simple.

[0008] Preferably, the method specifically includes the following steps:

[0009] S1. Pre-calcining a mixture containing a lithium source compound, a manganese source compound, an iron source compound, and a phosphorus source compound at a temperature of 300 to 380 ° C to obtain a precursor;

[0010] S2. allowing the precursor to adsorb a first modifier, thereby obtaining a modified precursor intermediate; wherein the first modifier contains at least one modifying functional group, the modifying functional group being selected from at least one of a hydroxyl group and a carboxyl group;

[0011] S3. Mixing a second modifier containing a hydroxyl group and / or a carboxyl group with the modified precursor intermediate, causing an esterification reaction between the second modifier and the modified precursor intermediate to obtain an ionic precursor; wherein the second modifier is used to provide an ionic polymer;

[0012] S4. Mixing the ionic precursor with the carbon source material, and then calcining the resulting mixture under a protective atmosphere. In the present invention, pre-calcining the raw materials for preparing the lithium manganese iron phosphate positive electrode material under the above-mentioned temperature conditions can improve the surface structure of the precursor, which is conducive to allowing the surface of the precursor to fully adsorb modified functional groups containing carboxyl (-COOH) or hydroxyl (-OH) after the addition of the first modifier, providing a prerequisite for the subsequent ionic polymer to be evenly and tightly attached to the surface of the precursor.

[0013] Preferably, the raw materials for preparing lithium manganese iron phosphate positive electrode materials include lithium source materials, manganese source materials, phosphorus source materials, and iron source materials; wherein the lithium source compounds include Li2CO3, CH3COOLi, and LiOH, the manganese source compounds include Mn(CH3COO)2, MnCO3, and MnSO4, the phosphorus source compounds include NH4H2PO4, (NH4)2HPO4, and (NH4)3PO4, and the iron source compounds include FeSO4i7H2O, FeC2O4·2H2O, and Fe(NO3)3·9H2O.

[0014] Preferably, the amount of the carbon source material added is 2% to 10% of the mass of the precursor.

[0015] Preferably, the carbon source material is glucose or sucrose.

[0016] Preferably, the first modifier includes at least one of thioglycolic acid, 3-mercaptopropionic acid, mercaptopropionic acid, mercaptoethanol, mercaptopropanol, and 2-mercaptoethanol; the second modifier includes at least one of 1,2-dimethyl-3-hydroxyethylimidazole p-toluenesulfonate, 1,2-dimethyl-3-hydroxyethylimidazole bis(trifluoromethanesulfonyl)imide, 1,2-dimethyl-3-hydroxyethylimidazole hexafluorophosphate, 1-carboxyethyl-3-methylimidazole bis(trifluoromethanesulfonyl)imide, 1-carboxyethyl-3-methylimidazole nitrate, and 1-carboxyethyl-3-methylimidazole hydrogen sulfate. The use of the above-mentioned first modifier to modify the precursor is conducive to the precursor fully adsorbing the modified functional groups containing -COOH or -OH, so that the -COOH or -OH groups are evenly spread along the surface of the precursor, providing a basis for the subsequent ionic polymer to be evenly and densely loaded on the surface of the precursor; at the same time, the use of the above-mentioned second modifier is conducive to the synthesis of specific ionic polymers attached to the surface of the precursor, and then utilizing the characteristics of the high dispersion and large loading area of ​​the ionic polymer to coat the surface of the precursor with a complete and uniform carbon coating layer.

[0017] Preferably, the first modifier is selected from one of thioglycolic acid and mercaptoethanol.

[0018] Preferably, the amount of the first modifier added to S2 is determined according to the ratio of precursor mass to first modifier mass = 1-2:1-2. For example, the amount of the first modifier added to S2 is determined according to the ratio of precursor mass to first modifier mass = 1:1, 1:2, or 2:1. By controlling the amount of the first modifier added, the surface modification effect of the first modifier on the precursor can be improved, allowing the precursor surface to fully adsorb the modified functional groups containing -COOH or -OH, so that the modified functional groups are evenly, densely, and completely spread along the surface of the precursor.

[0019] Preferably, the feeding amount of the first modifier in S2 is determined according to the mass of the precursor: the mass of the first modifier = 1:1.

[0020] Preferably, the concentration of the first modifier is 0.8 to 1.5 mol / L.

[0021] Preferably, the amount of the second modifier added in S3 is determined according to the mass of the second modifier: the mass of the first modifier = 1 to 2: 1 to 2. For example, the amount of the second modifier added in S3 is determined according to the mass of the second modifier: the mass of the first modifier = 1:1, 1:2 or 2:1. By controlling the amount of the second modifier added, the matching effect of the second modifier and the first modifier can be improved, and the efficiency and success rate of the esterification reaction between the second modifier and the modified functional group on the surface of the first modifier can be improved, which provides a basis for the subsequent ionic polymer to be evenly and tightly attached to the surface of the precursor through ester bonds, thereby facilitating the formation of a structurally stable, uniformly dispersed, dense and complete carbon coating layer on the surface of the positive electrode material after the subsequent addition of a carbon source.

[0022] Preferably, the feeding amount of the second modifier in S3 is determined according to the mass of the second modifier: the mass of the first modifier = 1:1.

[0023] Preferably, in S3, the temperature of the esterification reaction is 90 to 130° C., and the time is 20 to 90 minutes. For example, in S3, the temperature of the esterification reaction is 90° C., 100° C., 110° C., 120° C., or 130° C., and the time is 20 minutes, 30 minutes, 40 minutes, 50 minutes, 60 minutes, 70 minutes, 80 minutes, or 90 minutes. In the present invention, by controlling the temperature and time of the esterification reaction, the modified precursor intermediate and the second modifier are fully esterified under suitable conditions, thereby improving the success rate and efficiency of the ionic polymer formed thereby being loaded on the precursor surface, thereby facilitating the uniform coating of the subsequent carbon coating layer.

[0024] Preferably, the catalyst for the esterification reaction is dicyclohexylcarbodiimide.

[0025] Preferably, during the calcination process, the calcination system is first heated to a pre-calcination temperature of 270-320°C, and then the calcination system is heated to 630-670°C at a heating rate of no more than 2°C / min and kept at that temperature for 3-5 hours. For example, during the calcination process, the calcination system is first heated to a pre-calcination temperature of 270°C, 280°C, 290°C, 300°C, 310°C, or 320°C, and then the calcination system is heated to 630°C, 640°C, 650°C, 660°C, or 670°C at a heating rate of 1°C / min or 2°C / min, and kept at that temperature for 3, 4, or 5 hours. During the calcination process, controlling the calcination temperature, time and heating rate within this range is beneficial to controlling the crystal form of the carbon coating layer, avoiding excessive size growth or agglomeration of carbon particles during sintering, and thus forming a structurally stable, uniformly dispersed, dense and complete carbon coating layer on the surface of the positive electrode material, avoiding contact between the positive electrode material and the electrolyte, inhibiting the dissolution of manganese elements, and thereby improving the electrochemical properties and cycle performance of the carbon-coated lithium manganese iron phosphate positive electrode material.

[0026] Preferably, during the calcination process: first, the calcination system is heated to a pre-calcination temperature of 300° C., and then the calcination system is heated to 650° C. at a heating rate of 1° C. / min and kept at that temperature for 4 hours.

[0027] According to another aspect of the present invention, a carbon-coated lithium manganese iron phosphate positive electrode material is provided, which is prepared by the above-mentioned method for preparing the carbon-coated lithium manganese iron phosphate positive electrode material. DETAILED DESCRIPTION

[0028] In order to enable those skilled in the art to better understand the technical solutions of the present invention, the technical solutions of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. Obviously, the embodiments described are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts should fall within the scope of protection of the present invention.

[0029] Example 1

[0030] Treatment group 1

[0031] This treatment group provides a method for preparing a carbon-coated lithium manganese iron phosphate positive electrode material, comprising the following steps:

[0032] S1, lithium source, phosphorus source, manganese source and iron source according to the molar ratio of 1:1:x:(1-x)(0 <x<1,本处理组1中x=0.5),加入到无水乙醇介质中球磨6小时,干燥后所得粉末于管式炉中煅烧,煅烧温度为350℃,煅烧时间为5h,冷却后得到前驱体;其中,制备磷酸锰铁锂正极材料的原料由锂源化合物LiOH、锰源化合物MnCO3、磷源化合物NH4H2PO4、铁源化合物Fe(NO3)3·9H2O组成。

[0033] S2.1. Dispersing the precursor obtained in step S1 in an aqueous solution of a first modifier (thioglycolic acid) to allow the precursor to fully adsorb the first modifier to achieve carboxylation surface modification of the precursor by the first modifier, thereby obtaining a modified precursor intermediate; wherein the concentration of the first modifier is 0.9 mol / L, and the mass ratio of the first modifier to the precursor is 1:1;

[0034] S3, adding a second modifier (1,2-dimethyl-3-hydroxyethyl imidazole p-toluenesulfonate) to the solution obtained after completing step S2, fully stirring and dispersing, and adding an esterification catalyst (dicyclohexylcarbodiimide) to carry out an esterification reaction, the esterification reaction temperature is 100° C., the time is 1 hour, and the second modifier containing a hydroxyl group is used to react with the -COOH on the surface of the first modified precursor to cause an ionic polymer to adhere to the surface of the precursor; after the reaction is completed, filtering and washing to obtain an ionic precursor; wherein, the mass ratio of the second modifier to the first modifier in step S2 is 1:1;

[0035] S4. The ionic precursor obtained in step S3 is mixed with the carbon source material (glucose) and added to deionized water, and dried while stirring at a temperature of 80°C; wherein the amount of the carbon source material added is 2% of the mass of the precursor in step 1; then the mixture obtained after drying is placed in a tubular furnace and calcined under a nitrogen atmosphere, wherein the specific operation of calcination is as follows: first, the calcination system is heated to a pre-calcination temperature of 300°C at a rate of 5°C / min, and then the calcination system is heated to 650°C at a rate of 1°C / min and kept warm for 4 hours; after the calcination is completed, the carbon-coated lithium manganese iron phosphate positive electrode material is obtained after cooling, grinding and sieving.

[0036] Treatment group 2

[0037] This treatment group prepared carbon-coated lithium manganese iron phosphate cathode material by referring to treatment group 1 in this embodiment. The difference between this treatment group and treatment group 1 in this embodiment is that the calcination temperature in the precursor preparation process of this treatment group is 260°C. Apart from the above differences, the materials and process operations used in this treatment group are strictly consistent with those of treatment group 1 in this embodiment.

[0038] Treatment group 3

[0039] This treatment group prepared carbon-coated lithium manganese iron phosphate cathode material by referring to Treatment Group 1 in Example 1. The difference between this treatment group and Treatment Group 1 in this embodiment is that the calcination temperature during the precursor preparation process in this treatment group is 300°C. Apart from these differences, the materials and process operations used in this treatment group are strictly consistent with those of Treatment Group 1 in this embodiment.

[0040] Treatment group 4

[0041] This treatment group prepared carbon-coated lithium manganese iron phosphate cathode material by referring to Treatment Group 1 in Example 1. The difference between this treatment group and Treatment Group 1 in this embodiment is that the calcination temperature during the precursor preparation process in this treatment group is 380°C. Apart from these differences, the materials and process operations used in this treatment group are strictly consistent with those of Treatment Group 1 in this embodiment.

[0042] Treatment group 5

[0043] This treatment group prepared carbon-coated lithium manganese iron phosphate cathode material by referring to Treatment Group 1 in Example 1. The difference between this treatment group and Treatment Group 1 in this embodiment is that the calcination temperature during the precursor preparation process in this treatment group is 420°C. Apart from the above differences, the materials and process operations used in this treatment group are strictly consistent with those of Treatment Group 1 in this embodiment.

[0044] Example 2

[0045] Treatment group 1

[0046] This treatment group prepared carbon-coated lithium manganese iron phosphate positive electrode material according to Treatment Group 1 in Example 1. The materials and process operations used in this treatment group were strictly consistent with those of Treatment Group 1 in Example 1.

[0047] Treatment group 2

[0048] This treatment group prepared carbon-coated lithium manganese iron phosphate cathode materials by referring to Treatment Group 1 in this embodiment. The difference between this treatment group and Treatment Group 1 in this embodiment is that the first modifier used in this treatment group during the process of causing the precursor to adsorb the first modifier is mercaptopropionic acid. Apart from these differences, the materials and process operations used in this treatment group were strictly consistent with those of Treatment Group 1 in this embodiment.

[0049] Treatment group 3

[0050] This treatment group prepared carbon-coated lithium manganese iron phosphate cathode materials by referring to Treatment Group 1 in this embodiment. The difference between this treatment group and Treatment Group 1 in this embodiment is that propionic acid was used as the first modifier during the adsorption of the first modifier on the precursor. Other than these differences, the materials and process operations used in this treatment group were strictly consistent with those of Treatment Group 1 in this embodiment.

[0051] Treatment group 4

[0052] This treatment group prepared carbon-coated lithium manganese iron phosphate cathode material by referring to Treatment Group 1 in this embodiment. The difference between this treatment group and Treatment Group 1 in this embodiment is that the second modifier used in this treatment group during the esterification reaction with the modified precursor intermediate is 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide salt. Aside from these differences, the materials and process operations used in this treatment group were strictly consistent with those of Treatment Group 1 in this embodiment.

[0053] Treatment group 5

[0054] This treatment group prepared carbon-coated lithium manganese iron phosphate cathode material by referring to Treatment Group 1 in this embodiment. The difference between this treatment group and Treatment Group 1 in this embodiment is that the second modifier used in this treatment group during the esterification reaction with the modified precursor intermediate is 1,2-dimethyl-3-hydroxyethylimidazolium tetrafluoroborate. Aside from these differences, the materials and process operations used in this treatment group were strictly consistent with those of Treatment Group 1 in this embodiment.

[0055] Example 3

[0056] Treatment group 1

[0057] This treatment group prepared carbon-coated lithium manganese iron phosphate positive electrode material according to Treatment Group 1 in Example 1. The materials and process operations used in this treatment group were strictly consistent with those of Treatment Group 1 in Example 1.

[0058] Treatment group 2

[0059] This treatment group prepared carbon-coated lithium manganese iron phosphate cathode material by referring to treatment group 1 in this embodiment. The difference between this treatment group and treatment group 1 in this embodiment is that the esterification reaction temperature in this treatment group is 80°C. Apart from the above differences, the materials and process operations used in this treatment group are strictly consistent with those of treatment group 1 in this embodiment.

[0060] Treatment group 3

[0061] This treatment group prepared carbon-coated lithium manganese iron phosphate cathode material by referring to treatment group 1 in this embodiment. The difference between this treatment group and treatment group 1 in this embodiment is that during the esterification reaction with the addition of ionic liquid, the esterification reaction temperature in this treatment group was 90°C. Apart from these differences, the materials and process operations used in this treatment group were strictly consistent with those of treatment group 1 in this embodiment.

[0062] Treatment group 4

[0063] This treatment group prepared carbon-coated lithium manganese iron phosphate cathode material by referring to treatment group 1 in this embodiment. The difference between this treatment group and treatment group 1 in this embodiment is that during the esterification reaction with the addition of ionic liquid in this treatment group, the esterification reaction temperature was 130°C. Apart from these differences, the materials and process operations used in this treatment group were strictly consistent with those of treatment group 1 in this embodiment.

[0064] Treatment group 5

[0065] This treatment group prepared carbon-coated lithium manganese iron phosphate cathode material by referring to treatment group 1 in this embodiment. The difference between this treatment group and treatment group 1 in this embodiment is that during the esterification reaction with the addition of ionic liquid, the esterification reaction temperature in this treatment group was 140°C. Apart from these differences, the materials and process operations used in this treatment group were strictly consistent with those of treatment group 1 in this embodiment.

[0066] Example 4

[0067] Treatment group 1

[0068] This treatment group prepared carbon-coated lithium manganese iron phosphate positive electrode material according to Treatment Group 1 in Example 1. The materials and process operations used in this treatment group were strictly consistent with those of Treatment Group 1 in Example 1.

[0069] Treatment group 2

[0070] This treatment group prepared carbon-coated lithium manganese iron phosphate positive electrode materials with reference to treatment group 1 in this embodiment. The difference between this treatment group and treatment group 1 is that during the calcination process after adding the carbon source material, the calcination system was first heated to 300°C at a heating rate of 5°C / min, and then heated to 650°C at a heating rate of 2°C / min and maintained at this temperature for 4 hours. Aside from the above differences, the materials and process operations used in this treatment group were strictly consistent with those of treatment group 1 in this embodiment.

[0071] Treatment group 3

[0072] This treatment group prepared carbon-coated lithium manganese iron phosphate positive electrode materials with reference to treatment group 1 in this embodiment. The difference between this treatment group and treatment group 1 is that after adding the carbon source material, during the calcination process, the calcination system was first heated to 300°C at a heating rate of 5°C / min, and then heated to 650°C at a heating rate of 5°C / min and maintained at this temperature for 4 hours. Aside from these differences, the materials and process operations used in this treatment group were strictly consistent with those of treatment group 1 in this embodiment.

[0073] Comparative Example 1

[0074] Comparative treatment group 1

[0075] The carbon-coated lithium manganese iron phosphate cathode material was prepared in the comparative treatment group as follows:

[0076] (1) The raw materials for preparing lithium manganese iron phosphate positive electrode materials were ball-milled in anhydrous ethanol medium for 6 hours. The resulting powder after drying was sintered in a tube furnace at a sintering temperature of 350° C. for 5 hours. The precursor was obtained after cooling;

[0077] (2) adding a carbon source to the precursor, ball milling with anhydrous ethanol as a medium, drying to obtain a powder, and sintering the powder in a tubular furnace under a nitrogen atmosphere to obtain a carbon-coated lithium manganese iron phosphate composite positive electrode material.

[0078] Comparative treatment group 2

[0079] The carbon-coated lithium manganese iron phosphate cathode material was prepared in the comparative treatment group as follows:

[0080] (1) The raw materials for preparing lithium manganese iron phosphate positive electrode materials were ball-milled in anhydrous ethanol medium for 6 hours. The resulting powder after drying was sintered in a tube furnace at a sintering temperature of 350° C. for 5 hours. The precursor was obtained after cooling;

[0081] (2) A carbon source and an ionic liquid (1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide) were added to the precursor, and the mixture was ball-milled with anhydrous ethanol as a medium. The powder was dried and sintered in a tubular furnace under a nitrogen atmosphere to obtain a carbon-coated lithium manganese iron phosphate composite positive electrode material.

[0082] Comparative treatment group 3

[0083] This comparative treatment group was prepared using the same method as Treatment Group 1 in Example 1 to prepare a carbon-coated lithium manganese iron phosphate cathode material. This comparative treatment group differed from Treatment Group 1 in Example 1 in that the second modifier used in this treatment group during the esterification reaction with the modified precursor intermediate was mercaptoethanol. Other than these differences, the materials and process used in this comparative treatment group were identical to those used in Treatment Group 1 in this Example.

[0084] Comparative treatment group 4

[0085] This comparative treatment group was prepared using the same method as Treatment Group 1 in Example 1 to prepare a carbon-coated lithium manganese iron phosphate cathode material. The difference between this comparative treatment group and Treatment Group 1 in Example 1 is that the second modifier is mercaptopropanol. Other than these differences, the materials and process used in this treatment group were strictly consistent with those used in Treatment Group 1 in this example.

[0086] Comparative treatment group 5

[0087] This comparative treatment group was prepared using the same method as Treatment Group 1 in Example 1 to prepare a carbon-coated lithium manganese iron phosphate cathode material. The difference between this comparative treatment group and Treatment Group 1 in Example 1 was that the calcination temperature in this treatment group was 590°C. Apart from these differences, the materials and process operations used in this treatment group were strictly consistent with those of Treatment Group 1 in this example.

[0088] Control treatment group 6

[0089] This comparative treatment group was prepared using the same method as Treatment Group 1 in Example 1 to prepare a carbon-coated lithium manganese iron phosphate cathode material. The difference between this comparative treatment group and Treatment Group 1 in Example 1 was that the calcination temperature in this treatment group was 620°C. Apart from these differences, the materials and process used in this treatment group were strictly consistent with those used in Treatment Group 1 in this example.

[0090] Control treatment group 7

[0091] This comparative treatment group was prepared using the same method as Treatment Group 1 in Example 1 to prepare a carbon-coated lithium manganese iron phosphate cathode material. The difference between this comparative treatment group and Treatment Group 1 in Example 1 was that the calcination temperature in this treatment group was 680°C. Apart from these differences, the materials and process operations used in this treatment group were strictly consistent with those of Treatment Group 1 in this example.

[0092] Control treatment group 8

[0093] This comparative treatment group was prepared using the same method as Treatment Group 1 in Example 1 to prepare a carbon-coated lithium manganese iron phosphate cathode material. The difference between this comparative treatment group and Treatment Group 1 in Example 1 was that the calcination temperature in this treatment group was 720°C. Apart from these differences, the materials and process used in this treatment group were strictly consistent with those used in Treatment Group 1 in this example.

[0094] Test Example 1

[0095] 1. Test subjects:

[0096] Coin-type batteries were prepared and tested using the carbon-coated lithium iron manganese phosphate cathode materials obtained in Examples 1-4 and Comparative Example 1. The batteries were prepared as follows: the carbon-coated lithium iron manganese phosphate cathode material was mixed with acetylene black and a PVDF binder in a ratio of 80:15:5. The mixture was evenly coated onto a current collector aluminum foil, dried, and punched to form the positive electrode. Metallic lithium served as the negative electrode. The 2025 coin-type batteries were assembled in an argon-filled glove box, following the order of positive electrode (electrode sheet), separator, electrolyte, negative electrode, and nickel foam. Finally, the batteries were sealed using a battery sealer.

[0097] 2. Test items:

[0098] (1) Charge and discharge capacity and capacity retention: The prepared battery was charged at a rate of 0.5C and discharged at a rate of 0.5C in the voltage range of 2.0 to 4.5V at 25°C for a full charge and discharge cycle test. After 100 cycles, the capacity retention and charge and discharge specific capacity were recorded.

[0099] (2) Powder conductivity: The test was performed using a Ruike micro powder resistivity tester. The equipment parameters were: pressure 200 kg, holding time: 60 s, current: 100 μA, and powder weight: 3 g.

[0100] 3. Test results:

[0101] Table 1 Performance test results of the battery of Example 1

[0102]

[0103] Table 2 Performance test results of the battery in Example 2

[0104]

[0105]

[0106] Table 3 Performance test results of the battery of Example 3

[0107]

[0108] Table 4 Performance test results of the battery of Example 4

[0109]

[0110] Table 5 Performance test results of the battery of Comparative Example 1

[0111]

[0112] The test results are shown in Tables 1-5. The performance test results of the comparative treatment group 1-8 corresponding to the treatment group 1 of Example 1 are compared. As shown in Table 5, the electrochemical performance and cycle performance of the battery obtained by the comparative treatment group 1 are obviously lower than those of the battery obtained by the treatment group 1 of Example 1; compared with the comparative treatment group 1 (without surface modification of the precursor, without introduction of the ionic polymer, and directly forming the carbon coating layer on the surface of the precursor), the preparation method provided by the application can form a dense and uniform carbon coating layer on the surface of the lithium manganese iron phosphate positive electrode material, thereby improving the electrochemical performance and cycle performance of the battery obtained thereby. As shown in Table 5, the electrochemical performance and cycle performance of the battery obtained by the comparative treatment group 2 are obviously lower than those of the battery obtained by the treatment group 1 of Example 1; compared with the comparative treatment group 2 (without surface modification of the precursor, and only introducing the ionic compound to adhere to the surface of the precursor), the preparation method provided by the application performs surface modification on the precursor, so that the ionic polymer is closely and uniformly adhered to the surface of the precursor, thereby making the carbon coating layer uniformly adhere to the surface of the ionic precursor, and the carbon coating layer formed thereby has excellent film forming properties, is dense and complete, and is uniformly distributed without agglomeration, thereby improving the electrochemical performance and cycle performance of the battery obtained thereby. As shown in Table 5, the electrochemical performance and cycle performance of the battery obtained by the comparative treatment group 3 and 4 are obviously lower than those of the battery obtained by the treatment group 1 of Example 1; compared with the comparative treatment group 3 and 4 (using a non-ionic compound as the second modifier, so that the polymer adhered to the surface of the precursor is not an ionic polymer), the preparation method provided by the application uses a specific second modifier to adhere an ionic polymer to the surface of the precursor, and uses the characteristics of the ionic polymer, such as high dispersity and large loading area, to make the carbon source added subsequently adhere to the surface of the ionic precursor, thereby forming a dense, complete and uniformly dispersed carbon coating layer, and thereby improving the electrochemical performance and cycle performance of the battery obtained thereby. As shown in Table 5, the electrochemical performance and cycle performance of the battery obtained by the comparative treatment group 5-8 are obviously lower than those of the battery obtained by the treatment group 1 of Example 1; compared with the comparative treatment group 5-8 (the calcination temperature when the mixture is calcined in a protective atmosphere is not set in the range of 630-670℃), the preparation method provided by the application can form a dense and uniform carbon coating layer on the surface of the positive electrode material by reasonably setting the calcination temperature, thereby improving the electrochemical performance and cycle performance of the battery obtained thereby.

[0113] The performance test results of treatment group 1 of Example 1 were compared with those of treatment groups 2 to 5. As shown in Table 1, under the same conditions for other materials and operations in preparing the battery, the calcination temperature of treatment groups 2 and 5 during the precursor preparation process was not set within the preferred range of 300 to 380°C, and the electrochemical performance and cycle performance of the batteries obtained were lower than those of the batteries prepared by treatment groups 1, 3, and 4. This shows that, compared with treatment groups 2 and 5, treatment groups 1, 3, and 4 improved the surface structure of the precursor by reasonably setting the calcination temperature during the precursor preparation process, facilitating the subsequent surface modification of the precursor, and providing a basis for the uniform and dense attachment of the ionic polymer to the surface of the precursor, thereby improving the electrochemical performance and cycle performance of the carbon-coated lithium manganese iron phosphate positive electrode material obtained thereby.

[0114] The performance test results of Treatment Group 1 of Example 2 were compared with those of Treatment Groups 2 to 5. As shown in Table 2, under the same conditions for preparing other materials and operations, the electrochemical performance and cycle performance of the batteries obtained in Treatment Groups 2 and 3 were lower than those of the battery prepared in Treatment Group 1, as the first modifier was not thioglycolic acid or mercaptoethanol during the process of allowing the precursor to adsorb the first modifier. Furthermore, the electrochemical performance and cycle performance of the batteries obtained in Treatment Groups 4 and 5 were lower than those of the battery prepared in Treatment Group 1, as the second modifier was not used in the esterification reaction between the modified precursor intermediate and the modified precursor intermediate. This shows that by using a specific first modifier in the process of allowing the precursor to adsorb the first modifier, it is beneficial to fully adsorb the modified functional groups on the surface of the precursor, providing a basis for the subsequent ionic polymer to be evenly and densely attached to the surface of the precursor; moreover, by using a specific second modifier in the process of using the second modifier to undergo esterification reaction with the modified precursor intermediate, it is beneficial to synthesize a specific ionic polymer attached to the surface of the precursor, and then utilize the characteristics of high dispersion and large loading area of ​​the ionic polymer to coat a complete and uniform carbon coating layer on the surface of the precursor, thereby improving the electrochemical properties and cycle performance of the carbon-coated lithium manganese iron phosphate positive electrode material obtained thereby.

[0115] The performance test results of treatment group 1 and treatment groups 2 to 5 of Example 3 were compared. As shown in Table 3, under the same conditions for preparing other materials and operations for the batteries, the esterification temperature of treatment groups 2 and 5 was not set within the range of 90 to 130°C during the esterification reaction, and the electrochemical performance and cycle performance of the batteries obtained were lower than those of the batteries prepared by treatment groups 1, 3, and 4. This shows that, compared with treatment groups 2 and 5, treatment groups 1, 3, and 4 reasonably set the esterification temperature during the esterification reaction, so that the modified precursor intermediate and the second modifier fully undergo esterification reaction under suitable conditions, thereby improving the success rate and efficiency of the ionic polymer formed thereby being loaded on the precursor surface, thereby facilitating the uniform coating of the subsequent carbon coating layer, and thus improving the electrochemical performance and cycle performance of the carbon-coated lithium manganese iron phosphate positive electrode material obtained thereby.

[0116] The performance test results of treatment group 1 of Example 4 were compared with those of treatment groups 2 to 3. As shown in Table 4, under the same conditions for preparing other materials and operations for the battery, in treatment groups 2 and 3, after adding the carbon source material and calcining the mixture at a heating rate higher than 1°C / min, the electrochemical performance and cycle performance of the batteries obtained were lower than those of the battery prepared in treatment group 1. This shows that, compared with treatment groups 2 to 3, treatment group 1 is beneficial for controlling the crystal form of the carbon coating layer by reasonably setting the heating method and heating rate during the calcination process, avoiding excessive growth or agglomeration of carbon particles during the sintering process, thereby forming a uniformly dispersed, dense and complete carbon coating layer on the surface of the positive electrode material, thereby making the obtained lithium manganese iron phosphate positive electrode material have excellent electrochemical performance and cycle performance.

[0117] The above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit the scope of protection of the present invention. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that the technical solutions of the present invention may be modified or replaced by equivalents, but these modifications or replacements are all within the scope of protection of the present invention.

Claims

1. A method for preparing a carbon-coated lithium manganese iron phosphate positive electrode material, characterized in that: The steps include: A precursor containing lithium, manganese, iron, and phosphorus is prefabricated, and an ionic polymer is attached to the surface of the precursor by surface modification, thereby obtaining an ionic precursor. Specifically, the surface modification comprises: dispersing the precursor in a first modifier to obtain a modified precursor intermediate, mixing a second modifier with the modified precursor intermediate, and causing the second modifier and the modified precursor intermediate to undergo an esterification reaction, thereby obtaining the ionic precursor; The ionic precursor is mixed with a carbon source material, and the mixture is then calcined under a protective atmosphere. During the calcination process, the calcination system is heated to 630-670° C. and kept at this temperature for 3-5 hours to obtain the carbon-coated lithium manganese iron phosphate positive electrode material; The first modifier includes at least one of thioglycolic acid, 3-mercaptopropionic acid, mercaptopropionic acid, mercaptoethanol, mercaptopropanol, and 2-mercaptoethanol; the second modifier includes at least one of 1,2-dimethyl-3-hydroxyethylimidazolium p-toluenesulfonate, 1,2-dimethyl-3-hydroxyethylimidazolium bis(trifluoromethanesulfonyl)imide, 1,2-dimethyl-3-hydroxyethylimidazolium hexafluorophosphate, 1-carboxyethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide, 1-carboxyethyl-3-methylimidazolium nitrate, and 1-carboxyethyl-3-methylimidazolium hydrogen sulfate.

2. The method for preparing the carbon-coated lithium manganese iron phosphate positive electrode material according to claim 1, wherein: In the ionic precursor, the ionic polymer is bonded to the surface of the precursor via an ester bond.

3. The method for preparing the carbon-coated lithium manganese iron phosphate positive electrode material according to claim 2, wherein: The specific steps include: S1. Pre-calcining a mixture containing a lithium source compound, a manganese source compound, an iron source compound, and a phosphorus source compound at a temperature of 300 to 380 ° C to obtain the precursor; S2. dispersing the precursor in the first modifier to obtain a modified precursor intermediate; S3. mixing the second modifier and the modified precursor intermediate, so that the second modifier and the modified precursor intermediate undergo an esterification reaction, thereby obtaining the ionic precursor; S4. Mixing the ionic precursor with the carbon source material, and then calcining the resulting mixture under a protective atmosphere to obtain the carbon-coated lithium manganese iron phosphate positive electrode material.

4. The method for preparing the carbon-coated lithium manganese iron phosphate positive electrode material according to claim 1, wherein: The first modifier is selected from one of thioglycolic acid and mercaptoethanol.

5. The method for preparing the carbon-coated lithium manganese iron phosphate positive electrode material according to claim 3, wherein: The feeding amount of the first modifier in S2 is determined according to the mass of the precursor: the mass of the first modifier = 1-2:1-2.

6. The method for preparing the carbon-coated lithium manganese iron phosphate positive electrode material according to claim 3, wherein: The feeding amount of the second modifier in S3 is determined according to the mass of the second modifier: the mass of the first modifier = 1-2:1-2.

7. The method for preparing the carbon-coated lithium manganese iron phosphate positive electrode material according to claim 3, characterized in that: In S3, the esterification reaction is carried out at a temperature of 90 to 130° C. and for a time of 20 to 90 minutes.

8. The method for preparing the carbon-coated lithium manganese iron phosphate positive electrode material according to any one of claims 1 to 7, characterized in that: During the calcination process, the calcination system is first heated to a pre-calcination temperature of 270-320° C., and then the calcination system is heated to 630-670° C. at a heating rate not exceeding 2° C. / min and maintained at that temperature for 3-5 hours.

9. A carbon-coated lithium manganese iron phosphate positive electrode material, characterized in that: The carbon-coated lithium manganese iron phosphate positive electrode material is prepared by the preparation method of any one of claims 1 to 8.

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