Carbon-coated lithium ferric manganese phosphate, preparation method thereof, positive plate and lithium ion battery

Abstract

The invention provides carbon-coated lithium ferric manganese phosphate and a preparation method thereof, a positive plate and a lithium ion battery. The preparation method comprises the following steps: S1, mixing raw materials including a ferromanganese bimetallic N-heterocyclic carbene complex, a lithium source and a phosphorus source, and sintering in protective gas to obtain carbon-rich coated lithium ferric manganese phosphate; and S2, performing carbon emission treatment on the carbon-rich coated lithium ferric manganese phosphate in an oxygen-containing atmosphere to obtain the carbon-coated lithium ferric manganese phosphate. By adopting the ferromanganese bimetallic N-heterocyclic carbene complex as an iron source, a manganese source and a carbon source, the mixing uniformity between an iron element and a manganese element is improved, so that the probability of metal ion dissolution in the charging and discharging process of lithium ferric manganese phosphate is reduced. The carbon-coated lithium ferric manganese phosphate formed by taking the ferromanganese bimetallic N-heterocyclic carbene complex as a manganese source, an iron source and a carbon source to react with a lithium source and a phosphorus source has relatively high rate capability and cycling stability.

Description

Technical Field

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

[0002] Lithium-ion batteries are developing rapidly in the market now, and the positive electrode material is the most important component in lithium-ion batteries. As one of the positive electrode materials of lithium-ion batteries, lithium iron manganese phosphate has become the focus of scholars due to its advantages such as good safety, high energy density and low cost. However, the low conductivity of lithium iron manganese phosphate and the dissolution of metal elements limit its commercial application in batches. Therefore, it is urgent to develop a lithium iron manganese phosphate material with excellent comprehensive performance to meet the market demand for lithium battery positive electrode materials. Summary of the invention

[0003] The main purpose of the present invention is to provide a carbon-coated lithium iron manganese phosphate and a preparation method thereof, a positive electrode sheet and a lithium ion battery to solve the problem of low conductivity and stability of lithium iron manganese phosphate in the prior art.

[0004] In order to achieve the above-mentioned purpose, according to one aspect of the present invention, a method for preparing carbon-coated lithium iron manganese phosphate is provided, and the preparation method comprises the following steps: step S1, mixing raw materials including manganese-iron bimetallic nitrogen heterocyclic carbene complex, lithium source and phosphorus source and sintering them in a protective gas to obtain carbon-rich coated lithium iron manganese phosphate; step S2, decarbonizing the carbon-rich coated lithium iron manganese phosphate in an oxygen-containing atmosphere to obtain carbon-coated lithium iron manganese phosphate.

[0005] The carbon-coated lithium iron manganese phosphate formed by reacting a manganese-iron bimetallic nitrogen heterocyclic carbene complex as a manganese source, an iron source and a carbon source with a lithium source and a phosphorus source has high rate performance and cycle stability. Since too high a carbon content is not conducive to improving the energy density of the carbon-coated lithium iron manganese phosphate, the carbon-rich coated lithium iron manganese phosphate is subjected to a carbon removal treatment in an oxygen-containing atmosphere to remove part of the carbon to form a carbon-coated lithium iron manganese phosphate.

[0006] Furthermore, the temperature of the above-mentioned sintering treatment is 600-900°C; and / or, the sintering treatment time is 5-10 hours; and / or, the temperature of the carbon removal treatment is 200-400°C; and / or, the carbon removal treatment time is 10-20 hours; and / or, the volume content of oxygen in the oxygen-containing atmosphere is not less than 21%.

[0007] Controlling the temperature and time of the sintering treatment within the above ranges helps improve the stability of lithium iron manganese phosphate. Controlling the temperature and time of the carbon removal treatment within the above ranges helps improve the rate performance and cycle stability of the carbon-coated lithium iron manganese phosphate. Controlling the volume content of oxygen in the oxygen-containing atmosphere within the above range helps improve the removal efficiency of excess carbon.

[0008] Furthermore, the molar ratio of the lithium element in the above-mentioned lithium source, the iron element in the manganese-iron bimetallic nitrogen heterocyclic carbene complex, the manganese element in the manganese-iron bimetallic nitrogen heterocyclic carbene complex and the phosphorus element in the phosphorus source is 1.01-1.10:0.3-0.6:0.4-0.7:1; and / or, the lithium source is selected from any one or more of lithium carbonate, lithium oxide and lithium hydroxide; and / or, the phosphorus source is monoammonium phosphate and / or phosphorus oxide.

[0009] Controlling the molar ratio of lithium in the lithium source, iron in the manganese-iron bimetallic nitrogen heterocyclic carbene complex, manganese in the manganese-iron bimetallic nitrogen heterocyclic carbene complex and phosphorus in the phosphorus source within the above range helps to improve the structural stability of the carbon-coated lithium iron manganese phosphate.

[0010] Furthermore, the preparation method of the above-mentioned manganese-iron bimetallic nitrogen heterocyclic carbene complex includes: step S11, mixing raw materials including nitrogen heterocyclic carbene ligands, silver oxide and organic solvents and performing a first reaction under light-proof conditions to obtain a reaction solution; step S12, adding iron powder and manganese powder to the reaction solution for a second reaction to obtain a manganese-iron bimetallic nitrogen heterocyclic carbene complex; wherein the total molar ratio of iron powder and manganese powder to the molar ratio of nitrogen heterocyclic carbene ligands is 1 to 3:1; and / or, the molar ratio of nitrogen heterocyclic carbene ligands to silver oxide is 1:0.5 to 2; and / or, the average particle size of iron powder does not exceed 50 μm; and / or, the average particle size of manganese powder does not exceed 50 μm; preferably, the average particle size of iron powder is 20 to 40 μm; and / or, the average particle size of manganese powder is 20 to 40 μm.

[0011] Controlling the total molar ratio of iron powder and manganese powder to the molar ratio of nitrogen heterocyclic carbene ligand within the above range helps to improve the stability of the carbon-coated lithium iron manganese phosphate finally formed. Controlling the molar ratio of nitrogen heterocyclic carbene ligand to silver oxide within the above range helps to improve the reaction efficiency while reducing the reaction cost. Controlling the average particle size of iron powder and manganese powder within the above range helps to improve the efficiency of the coordination reaction.

[0012] Furthermore, the temperature of the second reaction is 50-100° C.; and / or the time of the second reaction is 5-30 h; and / or the temperature of the first reaction is 25-50° C.; and / or the time of the first reaction is 2-6 h.

[0013] Controlling the temperature and time of the second reaction and the temperature and time of the first reaction within the above ranges helps to improve the efficiency of the reaction.

[0014] Further, the above-mentioned nitrogen heterocyclic carbene ligand is selected from any one or more of thiazole nitrogen heterocyclic carbene ligands, imidazolidine nitrogen heterocyclic carbene ligands, imidazole nitrogen heterocyclic carbene ligands and triazole nitrogen heterocyclic carbene ligands; preferably, the nitrogen heterocyclic carbene ligand is selected from any one or more of 1-(2,4,6-trimethylphenyl)-3-[(2S)-4-methylpentanoic acid]imidazolium chloride, 1-(2,4,6-trimethylphenyl)-3-[(2S)-4-methylpentanoic acid]imidazolium hexafluorophosphate, 1,3-bis(2,4,6-trimethylphenyl)imidazolium tetrafluoroborate and 4,5-dimethyl-3-(2-oxo-2-phenylethyl)thiazole chloride; and / or, the organic solvent is selected from any one or more of acetonitrile, acetone and diethyl ether.

[0015] Controlling the type of nitrogen heterocyclic carbene ligand within the above range is helpful to improve the structural stability of lithium iron manganese phosphate. Controlling the type of organic solvent within the above range is helpful to improve the generation efficiency of manganese iron bimetallic nitrogen heterocyclic carbene complex.

[0016] According to another aspect of the present invention, a carbon-coated lithium iron manganese phosphate is provided. The carbon-coated lithium iron manganese phosphate is prepared by the aforementioned preparation method.

[0017] The carbon-coated lithium iron manganese phosphate has high rate performance and cycle stability.

[0018] Furthermore, the mass proportion of carbon in the carbon-coated lithium iron manganese phosphate is 1.72% to 1.89%.

[0019] Controlling the mass proportion of carbon in the carbon-coated lithium iron manganese phosphate within the above range can help improve the conductivity of the carbon-coated lithium iron manganese phosphate and reduce the probability of side reactions between the lithium iron manganese phosphate and the electrolyte.

[0020] According to another aspect of the present invention, there is provided a positive electrode sheet, comprising a current collector and a positive electrode coating, wherein the positive electrode coating contains the aforementioned carbon-coated lithium iron manganese phosphate.

[0021] The positive electrode sheet has high rate performance and cycle stability.

[0022] According to another aspect of the present invention, a lithium-ion battery is provided, comprising a positive electrode sheet, an electrolyte and a negative electrode sheet, wherein the positive electrode sheet is the positive electrode sheet mentioned above.

[0023] The above lithium-ion battery has high rate performance and cycle stability.

[0024] Applying the technical solution of the present application, the present application adopts a manganese-iron bimetallic nitrogen heterocyclic carbene complex as an iron source, a manganese source and a carbon source. On the one hand, it helps to improve the uniformity of the mixing between the iron element and the manganese element, thereby helping to improve the structural stability of the lithium iron manganese phosphate, and then helping to reduce the probability of metal ion dissolution of the lithium iron manganese phosphate during the charging and discharging process; on the second hand, the nitrogen heterocyclic carbene has a strong complexing ability with the iron element and the manganese element, which helps to reduce the dissolution probability of the iron element and the manganese element in the process of preparing the carbon-coated lithium iron manganese phosphate, thereby helping to further improve the structural stability of the lithium iron manganese phosphate; on the third hand, it helps to improve the uniformity of carbon distribution on the surface of the lithium iron manganese phosphate, thereby helping to improve the conductivity of the carbon-coated lithium iron manganese phosphate and reduce the probability of side reactions between the lithium iron manganese phosphate and the electrolyte. Therefore, the carbon-coated lithium iron manganese phosphate formed by the reaction of the manganese-iron bimetallic nitrogen heterocyclic carbene complex as a manganese source, an iron source and a carbon source with a lithium source and a phosphorus source has a high rate performance and cycle stability. In addition, since too high carbon content is not conducive to improving the energy density of carbon-coated lithium iron manganese phosphate, the carbon-rich coated lithium iron manganese phosphate is subjected to carbon removal treatment in an oxygen-containing atmosphere to remove part of the carbon to form carbon-coated lithium iron manganese phosphate. DETAILED DESCRIPTION

[0025] It should be noted that, in the absence of conflict, the embodiments and features in the embodiments of the present application can be combined with each other. The present invention will be described in detail below in conjunction with the embodiments.

[0026] As analyzed in the background technology of this application, the prior art lithium iron manganese phosphate has the problem of low conductivity and stability. In order to solve this problem, this application provides a carbon-coated lithium iron manganese phosphate and a preparation method thereof, a positive electrode sheet and a lithium ion battery.

[0027] It should be noted that, in the present application, the carbon-rich in the carbon-rich coated lithium iron manganese phosphate means that the mass proportion of carbon in the carbon-rich coated lithium iron manganese phosphate material is more than 2%.

[0028] In a typical embodiment of the present application, a method for preparing carbon-coated lithium iron manganese phosphate is provided, and the preparation method comprises the following steps: step S1, mixing raw materials including a manganese-iron bimetallic nitrogen heterocyclic carbene complex, a lithium source and a phosphorus source, and then sintering them in a protective gas to obtain a carbon-rich coated lithium iron manganese phosphate; step S2, decarbonizing the carbon-rich coated lithium iron manganese phosphate in an oxygen-containing atmosphere to obtain a carbon-coated lithium iron manganese phosphate.

[0029] The present application adopts a manganese-iron bimetallic nitrogen heterocyclic carbene complex as an iron source, a manganese source and a carbon source. On the one hand, it helps to improve the uniformity of the mixing between the iron element and the manganese element, thereby helping to improve the structural stability of the lithium iron manganese phosphate, and then helping to reduce the probability of metal ion dissolution of the lithium iron manganese phosphate during the charging and discharging process; on the second hand, the nitrogen heterocyclic carbene has a strong complexing ability with the iron element and the manganese element, which helps to reduce the dissolution probability of the iron element and the manganese element in the process of preparing the carbon-coated lithium iron manganese phosphate, thereby helping to further improve the structural stability of the lithium iron manganese phosphate; on the third hand, it helps to improve the uniformity of carbon distribution on the surface of the lithium iron manganese phosphate, thereby helping to improve the conductivity of the carbon-coated lithium iron manganese phosphate and reduce the probability of side reactions between the lithium iron manganese phosphate and the electrolyte. Therefore, the carbon-coated lithium iron manganese phosphate formed by the reaction of the manganese-iron bimetallic nitrogen heterocyclic carbene complex as a manganese source, an iron source and a carbon source with a lithium source and a phosphorus source has a high rate performance and cycle stability. In addition, since too high carbon content is not conducive to improving the energy density of carbon-coated lithium iron manganese phosphate, the carbon-rich coated lithium iron manganese phosphate is subjected to carbon removal treatment in an oxygen-containing atmosphere to remove part of the carbon to form carbon-coated lithium iron manganese phosphate.

[0030] Including but not limited to, the above-mentioned protective gas is nitrogen or argon.

[0031] In one embodiment of the present application, the temperature of the above-mentioned sintering treatment is 600-900°C; and / or, the time of the sintering treatment is 5-10 hours; and / or, the temperature of the decarbonization treatment is 200-400°C; and / or, the time of the decarbonization treatment is 10-20 hours; and / or, the volume content of oxygen in the oxygen-containing atmosphere is not less than 21%.

[0032] It is preferred to control the temperature and time of the sintering treatment within the above range, which helps to improve the stability of the lithium iron manganese phosphate. It is preferred to control the temperature and time of the carbon removal treatment within the above range, which helps to remove excess carbon without affecting the stability of the lithium iron manganese phosphate, thereby helping to control the carbon content in the carbon-coated lithium iron manganese phosphate within a suitable range, thereby helping to improve the rate performance and cycle stability of the carbon-coated lithium iron manganese phosphate. It is preferred to control the volume content of oxygen in the oxygen-containing atmosphere within the above range, which helps to improve the removal efficiency of excess carbon.

[0033] In order to improve the structural stability of the carbon-coated lithium iron manganese phosphate, in one embodiment of the present application, the molar ratio of the lithium element in the above-mentioned lithium source, the iron element in the manganese-iron bimetallic nitrogen heterocyclic carbene complex, the manganese element in the manganese-iron bimetallic nitrogen heterocyclic carbene complex and the phosphorus element in the phosphorus source is preferably 1.01-1.10:0.3-0.6:0.4-0.7:1; and / or, the lithium source is selected from any one or more of lithium carbonate, lithium oxide and lithium hydroxide; and / or, the phosphorus source is monoammonium phosphate and / or phosphorus oxide.

[0034] It is preferred to control the types of lithium source and phosphorus source within the above range, which helps to reduce material costs and enrich material selectivity.

[0035] In one embodiment of the present application, the preparation method of the above-mentioned manganese-iron bimetallic nitrogen heterocyclic carbene complex includes: step S11, mixing raw materials including nitrogen heterocyclic carbene ligands, silver oxide and organic solvents and performing a first reaction under light-proof conditions to obtain a reaction solution; step S12, adding iron powder and manganese powder to the reaction solution for a second reaction to obtain a manganese-iron bimetallic nitrogen heterocyclic carbene complex; wherein the total molar ratio of iron powder and manganese powder to the molar ratio of nitrogen heterocyclic carbene ligands is 1 to 3:1; and / or, the molar ratio of nitrogen heterocyclic carbene ligands to silver oxide is 1:0.5 to 2; and / or, the average particle size of iron powder does not exceed 50μm; and / or, the average particle size of manganese powder does not exceed 50μm; preferably, the average particle size of iron powder is 20 to 40μm; and / or, the average particle size of manganese powder is 20 to 40μm.

[0036] In step S11, the nitrogen heterocyclic carbene ligand first reacts with the catalyst silver oxide to form an intermediate, thereby helping to reduce the activation energy required for the reaction, and in step S12, the iron powder and the manganese powder react with the intermediate to form a manganese-iron bimetallic nitrogen heterocyclic carbene complex. It is preferred to control the total mole ratio of the iron powder and the manganese powder to the nitrogen heterocyclic carbene ligand within the above range, which helps to control the iron element, the manganese element and the carbon element within a suitable range, thereby helping to improve the stability of the carbon-coated iron manganese lithium phosphate formed. It is preferred to control the mole ratio of the nitrogen heterocyclic carbene ligand to silver oxide within the above range, which helps to improve the reaction efficiency while reducing the reaction cost. The average particle size of the iron powder and the manganese powder is too high, which is not conducive to improving the activity of the iron powder and the manganese powder surface, and is not conducive to improving the reaction efficiency of the iron powder and the manganese powder with the nitrogen heterocyclic carbene ligand. The average particle size of the iron powder and the manganese powder is too low, and it is easy to agglomerate between the particles, which is not conducive to improving the uniformity of the reaction. It is preferred to control the average particle size of the iron powder and the manganese powder within the above range, which helps to improve the efficiency of the coordination reaction.

[0037] In order to improve the efficiency of the reaction and promote the formation of manganese-iron bimetallic nitrogen heterocyclic carbene complexes, in one embodiment of the present application, the temperature of the second reaction is preferably 50-100°C; and / or the time of the second reaction is 5-30h; and / or the temperature of the first reaction is 25-50°C; and / or the time of the first reaction is 2-6h.

[0038] In one embodiment of the present application, the above-mentioned nitrogen heterocyclic carbene ligand is selected from any one or more of thiazole nitrogen heterocyclic carbene ligands, imidazolidine nitrogen heterocyclic carbene ligands, imidazole nitrogen heterocyclic carbene ligands and triazole nitrogen heterocyclic carbene ligands; preferably, the nitrogen heterocyclic carbene ligand is selected from any one or more of 1-(2,4,6-trimethylphenyl)-3-[(2S)-4-methylpentanoic acid]imidazolium chloride, 1-(2,4,6-trimethylphenyl)-3-[(2S)-4-methylpentanoic acid]imidazolium hexafluorophosphate, 1,3-bis(2,4,6-trimethylphenyl)imidazolium tetrafluoroborate and 4,5-dimethyl-3-(2-oxo-2-phenylethyl)thiazole chloride; and / or, the organic solvent is selected from any one or more of acetonitrile, acetone and diethyl ether.

[0039] It is preferred to control the type of nitrogen heterocyclic carbene ligand within the above range, which helps to improve the complexation of nitrogen heterocyclic carbene with iron and manganese elements, thereby helping to improve the structural stability of lithium iron manganese phosphate. It is preferred to control the type of organic solvent within the above range, which helps to improve the solubility of nitrogen heterocyclic carbene ligand in organic solvent, thereby helping to improve the generation efficiency of manganese-iron bimetallic nitrogen heterocyclic carbene complex.

[0040] In another typical embodiment of the present application, a carbon-coated lithium iron manganese phosphate is provided, and the carbon-coated lithium iron manganese phosphate is prepared by the aforementioned preparation method.

[0041] Since the carbon-coated lithium iron manganese phosphate is obtained by the preparation method of the present application, the carbon-coated lithium iron manganese phosphate has higher rate performance and cycle stability.

[0042] In order to improve the conductivity of the carbon-coated lithium iron manganese phosphate and reduce the probability of side reactions between the lithium iron manganese phosphate and the electrolyte, in one embodiment of the present application, the mass proportion of carbon in the above-mentioned carbon-coated lithium iron manganese phosphate is preferably 1.72% to 1.89%.

[0043] In another typical embodiment of the present application, a positive electrode sheet is provided, including a current collector and a positive electrode coating, wherein the positive electrode coating contains the aforementioned carbon-coated lithium iron manganese phosphate.

[0044] Since the positive electrode sheet contains the carbon-coated lithium iron manganese phosphate obtained by the preparation method of the present application, the positive electrode sheet has higher rate performance and cycle stability.

[0045] In another typical embodiment of the present application, a lithium-ion battery is provided, comprising a positive electrode sheet, an electrolyte and a negative electrode sheet, wherein the positive electrode sheet is the positive electrode sheet mentioned above.

[0046] Since the above lithium-ion battery contains the positive electrode sheet of the present application, the lithium-ion battery has higher rate performance and cycle stability.

[0047] The beneficial effects of the present application will be further illustrated below in conjunction with embodiments.

[0048] Example 1

[0049] S1. Add 1 mol of 1-(2,4,6-trimethylphenyl)-3-[(2S)-4-methylpentanoic acid]imidazolium chloride and 1 mol of silver oxide to acetonitrile solvent, conduct a first reaction under light-proof conditions, the reaction temperature is 30° C., the reaction time is 4 hours, and then add 0.8 mol of iron powder and 1.2 mol of manganese powder (the average particle size of the iron powder and the manganese powder is independently 30 μm) to conduct a second reaction, the reaction temperature is 90° C., the reaction time is 12 hours, and after the reaction is completed, filter the reaction solution, add ether to the filtrate for crystallization, and obtain a manganese-iron bimetallic nitrogen heterocyclic carbene complex;

[0050] S2, adding the manganese-iron bimetallic nitrogen heterocyclic carbene complex to lithium carbonate and monoammonium phosphate to obtain a mixture, wherein the molar ratio of Li:Fe:Mn:P is 1.05:0.4:0.6:1, and sintering the mixture in nitrogen at a temperature of 850°C for 7 hours;

[0051] S3. The product in S2 is placed in the air for carbon removal treatment at a temperature of 350° C. for 10 hours to obtain carbon-coated lithium iron manganese phosphate, wherein the mass proportion of carbon in the carbon-coated lithium iron manganese phosphate is 1.74%.

[0052] Example 2

[0053] S1. Add 1 mol of 1-(2,4,6-trimethylphenyl)-3-[(2S)-4-methylpentanoic acid]imidazolium hexafluorophosphate and 1.5 mol of silver oxide to acetonitrile solvent, conduct a first reaction under light-shielding conditions, the reaction temperature is 50° C., the reaction time is 3 h, and then add 0.3 mol of iron powder and 0.7 mol of manganese powder (the average particle size of the iron powder and the manganese powder is independently 40 μm) to conduct a second reaction, the reaction temperature is 80° C., the reaction time is 25 hours, and after the reaction is completed, filter the reaction solution, add ether to the filtrate for crystallization, and obtain a manganese-iron bimetallic nitrogen heterocyclic carbene complex;

[0054] S2, adding manganese iron bimetallic nitrogen heterocyclic carbene complex to lithium hydroxide and phosphorus oxide to obtain a mixture, wherein the molar ratio of Li:Fe:Mn:P is 1.08:0.3:0.7:1, and sintering the mixture in nitrogen at a temperature of 650°C for 9 hours;

[0055] S3. Place the product in S2 in the air for carbon removal treatment at a temperature of 400° C. for 12 hours to obtain carbon-coated lithium iron manganese phosphate.

[0056] Example 3

[0057] S1. Add 1 mol of 1,3-bis(2,4,6-trimethylphenyl)imidazolium tetrafluoroborate and 1.5 mol of silver oxide to acetonitrile solvent, conduct a first reaction under light-proof conditions, the reaction temperature is 25° C., the reaction time is 2 h, and then add 1.5 mol of iron powder and 1.5 mol of manganese powder (the average particle size of the iron powder and the manganese powder is independently 25 μm) to conduct a second reaction, the reaction temperature is 70° C., and the reaction time is 26 hours. After the reaction is completed, filter the reaction solution, add ether to the filtrate for crystallization, and obtain a manganese-iron bimetallic nitrogen heterocyclic carbene complex;

[0058] S2, adding the manganese-iron bimetallic nitrogen heterocyclic carbene complex to lithium carbonate and phosphorus oxide to obtain a mixture, wherein the molar ratio of Li:Fe:Mn:P is 1.1:0.5:0.5:1, and sintering the mixture in nitrogen at a temperature of 750°C for 6 hours;

[0059] S3. The product in S2 is placed in air for carbon removal treatment at a temperature of 250° C. for 20 h to obtain carbon-coated lithium iron manganese phosphate.

[0060] Example 4

[0061] S1. Add 1 mol of 4,5-dimethyl-3-(2-oxo-2-phenylethyl)thiazole chloride and 0.5 mol of silver oxide to acetonitrile solvent, carry out a first reaction under light-proof conditions, the reaction temperature is 30° C., the reaction time is 3 h, and then add 0.4 mol of iron powder and 0.6 mol of manganese powder (the average particle size of the iron powder and the manganese powder is independently 20 μm) to carry out a second reaction, the reaction temperature is 90° C., the reaction time is 8 hours, after the reaction is completed, filter the reaction solution, add ether to the filtrate for crystallization, and obtain a manganese-iron bimetallic nitrogen heterocyclic carbene complex;

[0062] S2, adding manganese iron bimetallic nitrogen heterocyclic carbene complex to lithium oxide and monoammonium phosphate to obtain a mixture, wherein the molar ratio of Li:Fe:Mn:P is 1.02:0.4:0.6:1, and sintering the mixture in nitrogen at a temperature of 650°C for 10 hours;

[0063] S3. The product in S2 is placed in air for carbon removal treatment at a temperature of 320°C for 17 hours to obtain carbon-coated lithium iron manganese phosphate.

[0064] Example 5

[0065] The difference from Example 1 is that the average particle size of the iron powder and the manganese powder is independently 40 μm, and carbon-coated lithium iron manganese phosphate is finally obtained.

[0066] Example 6

[0067] The difference from Example 1 is that the average particle size of the iron powder and the manganese powder is independently 50 μm, and carbon-coated lithium iron manganese phosphate is finally obtained.

[0068] Example 7

[0069] The difference from Example 1 is that the average particle size of the iron powder and the manganese powder is independently 60 μm, and carbon-coated lithium iron manganese phosphate is finally obtained.

[0070] Example 8

[0071] The difference from Example 1 is that the temperature of the first reaction is 25° C., the time of the first reaction is 6 hours, the temperature of the second reaction is 100° C., the time of the second reaction is 5 hours, and finally carbon-coated lithium iron manganese phosphate is obtained.

[0072] Example 9

[0073] The difference from Example 1 is that the temperature of the first reaction is 50° C., the time of the first reaction is 2 h, the temperature of the second reaction is 50° C., the time of the second reaction is 30 h, and finally carbon-coated lithium iron manganese phosphate is obtained.

[0074] Example 10

[0075] The difference from Example 1 is that the temperature of the first reaction is 20° C., the time of the first reaction is 76 h, the temperature of the second reaction is 110° C., the time of the second reaction is 4 h, and finally carbon-coated lithium iron manganese phosphate is obtained.

[0076] Embodiment 11

[0077] The difference from Example 1 is that the sintering temperature is 600°C, the sintering time is 10 hours, the carbon removal temperature is 200°C, and the carbon removal time is 20 hours, and finally the carbon-coated lithium iron manganese phosphate is obtained.

[0078] Example 12

[0079] The difference from Example 1 is that the sintering temperature is 900° C., the sintering time is 5 h, the carbon removal temperature is 400° C., the carbon removal time is 10 h, and finally carbon-coated lithium iron manganese phosphate is obtained.

[0080] Example 13

[0081] The difference from Example 1 is that the sintering temperature is 950° C., the sintering time is 4 h, the carbon removal temperature is 150° C., the carbon removal time is 25 h, and finally carbon-coated lithium iron manganese phosphate is obtained.

[0082] Embodiment 14

[0083] The difference from Example 1 is that the molar number of iron powder is 0.4 mol, the molar number of manganese powder is 0.6 mol, and finally carbon-coated lithium iron manganese phosphate is obtained, wherein the mass proportion of carbon in the carbon-coated lithium iron manganese phosphate is 1.89%.

[0084] Embodiment 15

[0085] The difference from Example 1 is that the molar number of iron powder is 1.2 mol, the molar number of manganese powder is 1.8 mol, and finally carbon-coated lithium iron manganese phosphate is obtained, wherein the mass proportion of carbon in the carbon-coated lithium iron manganese phosphate is 1.72%.

[0086] Example 16

[0087] The difference from Example 1 is that the molar number of iron powder is 0.2 mol, the molar number of manganese powder is 0.3 mol, and finally carbon-coated lithium iron manganese phosphate is obtained, wherein the mass proportion of carbon in the carbon-coated lithium iron manganese phosphate is 2.26%.

[0088] Comparative Example 1

[0089] S1. Add 1 mol of 1-(2,4,6-trimethylphenyl)-3-[(2S)-4-methylpentanoic acid]imidazolium chloride and 1 mol of silver oxide to acetonitrile solvent, conduct a first reaction under light-proof conditions, the reaction temperature is 30° C., the reaction time is 4 hours, and then add 0.8 mol of iron powder (the average particle size of the iron powder is 30 μm) to conduct a second reaction, the reaction temperature is 90° C., the reaction time is 12 hours, and after the reaction is completed, the reaction solution is filtered, and diethyl ether is added to the filtrate for crystallization to obtain an iron-metal nitrogen heterocyclic carbene complex;

[0090] S2, adding the iron metal nitrogen heterocyclic carbene complex and 1.2 mol of manganese powder (the average particle size of the manganese powder is 30 μm) to lithium carbonate and monoammonium phosphate to obtain a mixture, wherein the molar ratio of Li:Fe:Mn:P is 1.05:0.4:0.6:1, and sintering the mixture in nitrogen at a temperature of 850°C for 7 hours;

[0091] S3. The product in S2 is placed in air for carbon removal treatment at a temperature of 350° C. for 10 hours to obtain carbon-coated lithium iron manganese phosphate.

[0092] Comparative Example 2

[0093] S1. 1 mol of 1-(2,4,6-trimethylphenyl)-3-[(2S)-4-methylpentanoic acid]imidazolium chloride, 0.8 mol of iron powder and 1.2 mol of manganese powder (the average particle size of the iron powder and the manganese powder is 30 μm respectively) are added to lithium carbonate and monoammonium phosphate to obtain a mixture, wherein the molar ratio of Li:Fe:Mn:P is 1.05:0.4:0.6:1, and the mixture is sintered in nitrogen at a temperature of 850°C for 7 hours;

[0094] S2. The product in S1 is placed in air for carbon removal treatment at a temperature of 350° C. for 10 h to obtain carbon-coated lithium iron manganese phosphate.

[0095] Comparative Example 3

[0096] The difference from Comparative Example 2 is that glucose is used to replace 1-(2,4,6-trimethylphenyl)-3-[(2S)-4-methylpentanoic acid]imidazolium chloride, and carbon-coated lithium iron manganese phosphate is finally obtained.

[0097] The carbon-coated lithium iron manganese phosphate prepared in the above embodiment and comparative example was used as the positive electrode material, lithium sheet was used as the negative electrode material, and lithium hexafluorophosphate was used as the main component of the electrolyte for button assembly. The electrochemical window of charge and discharge was 2.5-4.5V. The specific capacity at 0.2C and 1C rates and the capacity retention rate after 600 cycles at 1C were tested. The test results are shown in Table 1.

[0098] Table 1

[0099]

[0100]

[0101] From the above description, it can be seen that the above embodiments of the present invention achieve the following technical effects:

[0102] The present application adopts a manganese-iron bimetallic nitrogen heterocyclic carbene complex as an iron source, a manganese source and a carbon source. On the one hand, it helps to improve the uniformity of the mixing between the iron element and the manganese element, thereby helping to improve the structural stability of the lithium iron manganese phosphate, and then helping to reduce the probability of metal ion dissolution of the lithium iron manganese phosphate during the charging and discharging process; on the second hand, the nitrogen heterocyclic carbene has a strong complexing ability with the iron element and the manganese element, which helps to reduce the dissolution probability of the iron element and the manganese element in the process of preparing the carbon-coated lithium iron manganese phosphate, thereby helping to further improve the structural stability of the lithium iron manganese phosphate; on the third hand, it helps to improve the uniformity of carbon distribution on the surface of the lithium iron manganese phosphate, thereby helping to improve the conductivity of the carbon-coated lithium iron manganese phosphate and reduce the probability of side reactions between the lithium iron manganese phosphate and the electrolyte. Therefore, the carbon-coated lithium iron manganese phosphate formed by the reaction of the manganese-iron bimetallic nitrogen heterocyclic carbene complex as a manganese source, an iron source and a carbon source with a lithium source and a phosphorus source has a high rate performance and cycle stability. In addition, since too high carbon content is not conducive to improving the energy density of carbon-coated lithium iron manganese phosphate, the carbon-rich coated lithium iron manganese phosphate is subjected to carbon removal treatment in an oxygen-containing atmosphere to remove part of the carbon to form carbon-coated lithium iron manganese phosphate.

[0103] The above are only preferred embodiments of the present invention and are not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A method for preparing carbon-coated lithium iron manganese phosphate, characterized in that: The preparation method comprises the following steps: Step S1, mixing raw materials including a manganese-iron bimetallic nitrogen heterocyclic carbene complex, a lithium source and a phosphorus source, and then sintering them in a protective gas to obtain carbon-rich coated lithium iron manganese phosphate; Step S2, subjecting the carbon-rich coated lithium iron manganese phosphate to a carbon removal treatment in an oxygen-containing atmosphere to obtain the carbon-coated lithium iron manganese phosphate.

2. The preparation method according to claim 1, characterized in that: The sintering temperature is 600-900°C; and / or, the sintering time is 5-10 hours; and / or, the decarbonization temperature is 200-400°C; and / or, the decarbonization time is 10-20 hours; and / or, the volume content of oxygen in the oxygen-containing atmosphere is not less than 21%.

3. The preparation method according to claim 1 or 2, characterized in that: The molar ratio of the lithium element in the lithium source, the iron element in the manganese-iron bimetallic nitrogen heterocyclic carbene complex, the manganese element in the manganese-iron bimetallic nitrogen heterocyclic carbene complex and the phosphorus element in the phosphorus source is 1.01-1.10:0.3-0.6:0.4-0.7:1; And / or, the lithium source is selected from any one or more of lithium carbonate, lithium oxide and lithium hydroxide; And / or, the phosphorus source is monoammonium phosphate and / or phosphorus oxide.

4. The preparation method according to any one of claims 1 to 3, characterized in that The preparation method of the manganese-iron bimetallic nitrogen heterocyclic carbene complex comprises: Step S11, mixing raw materials including a nitrogen heterocyclic carbene ligand, silver oxide and an organic solvent, and performing a first reaction under light-shielding conditions to obtain a reaction solution; Step S12, adding iron powder and manganese powder to the reaction solution to carry out a second reaction to obtain a manganese-iron bimetallic nitrogen heterocyclic carbene complex; Wherein, the total molar ratio of the iron powder and the manganese powder to the molar ratio of the nitrogen heterocyclic carbene ligand is 1 to 3:1; and / or, the molar ratio of the nitrogen heterocyclic carbene ligand to the silver oxide is 1:0.5-2; And / or, the average particle size of the iron powder does not exceed 50 μm; and / or, the average particle size of the manganese powder does not exceed 50 μm; preferably, the average particle size of the iron powder is 20-40 μm; and / or, the average particle size of the manganese powder is 20-40 μm.

5. The preparation method according to claim 4, characterized in that: The temperature of the second reaction is 50-100° C.; and / or, the time of the second reaction is 5-30 hours; and / or, the temperature of the first reaction is 25-50° C.; and / or, the time of the first reaction is 2-6 hours.

6. The preparation method according to claim 4 or 5, characterized in that: The nitrogen heterocyclic carbene ligand is selected from any one or more of thiazole nitrogen heterocyclic carbene ligands, imidazolidine nitrogen heterocyclic carbene ligands, imidazole nitrogen heterocyclic carbene ligands and triazole nitrogen heterocyclic carbene ligands; Preferably, the nitrogen heterocyclic carbene ligand is selected from any one or more of 1-(2,4,6-trimethylphenyl)-3-[(2S)-4-methylpentanoate]imidazolium chloride, 1-(2,4,6-trimethylphenyl)-3-[(2S)-4-methylpentanoate]imidazolium hexafluorophosphate, 1,3-bis(2,4,6-trimethylphenyl)imidazolium tetrafluoroborate and 4,5-dimethyl-3-(2-oxo-2-phenylethyl)thiazole chloride; And / or, the organic solvent is selected from any one or more of acetonitrile, acetone and diethyl ether.

7. A carbon-coated lithium iron manganese phosphate, characterized in that: The carbon-coated lithium iron manganese phosphate is prepared by the preparation method described in any one of claims 1 to 6.

8. The carbon-coated lithium iron manganese phosphate according to claim 7, characterized in that: The mass proportion of carbon in the carbon-coated lithium iron manganese phosphate is 1.72% to 1.89%.

9. A positive electrode sheet, comprising a current collector and a positive electrode coating, characterized in that: The positive electrode coating contains the carbon-coated lithium iron manganese phosphate according to claim 7 or 8.

10. A lithium ion battery comprising a positive electrode, an electrolyte and a negative electrode, characterized in that: The positive electrode sheet is the positive electrode sheet according to claim 9.

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