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

Abstract

The invention provides a carbon-coated lithium manganese iron phosphate material 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 manganese source, an iron source and an alkali solution, and performing precipitation reaction to obtain an iron-manganese compound; s2, mixing an iron-manganese compound and a carbon source A, and sequentially performing pre-sintering treatment and reduction sintering treatment to obtain a reduction sintering product; and S3, in protective gas, mixing the sintered product, a lithium source, a phosphorus source and a carbon source B, and calcining to obtain the carbon-coated lithium iron manganese phosphate material. The iron-manganese compound and the carbon source A are mixed and then sequentially subjected to pre-sintering treatment and reduction sintering treatment, so that a stable carbon-coated structure is favorably formed, reduction of metal ions is promoted, dissolution of the metal ions is favorably reduced, and the structural stability of the material is enhanced. The carbon-coated lithium manganese iron phosphate material has relatively high specific capacity and cycling stability.

Description

Technical Field

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

[0002] Currently, the development of lithium-ion batteries in the market is rapid, and the positive electrode material is the most important component in lithium-ion batteries. As one of the positive electrode materials for lithium-ion batteries, lithium iron manganese phosphate has the advantages of good safety, high energy density, low cost, etc., and has become the focus of attention of scholars. However, the low conductivity of lithium iron manganese phosphate and the dissolution of metal elements limit its large-scale commercial application, and the sintering temperature of lithium iron manganese phosphate is usually relatively high, resulting in large energy consumption. Even though carbon coating has been used in the prior art to improve the conductivity of lithium iron manganese phosphate and reduce the dissolution of metal elements, the specific capacity and cycle stability of the carbon-coated lithium iron manganese phosphate material still need to be improved. Therefore, there is an urgent need 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 object of the present invention is to provide a carbon-coated lithium iron manganese phosphate material, a preparation method thereof, a positive electrode sheet and a lithium-ion battery, so as to solve the problem of relatively low specific capacity and cycle stability of the carbon-coated lithium iron manganese phosphate material in the prior art.

[0004] To achieve the above object, according to one aspect of the present invention, a preparation method of a carbon-coated lithium iron manganese phosphate material is provided. The preparation method includes the following steps: Step S1, mixing raw materials including a manganese source, an iron source and an alkali solution, and performing a precipitation reaction to obtain an iron manganese compound; Step S2, mixing the iron manganese compound and a carbon source A, and successively performing a pre-sintering treatment and a reduction sintering treatment to obtain a reduction sintering product; Step S3, in a protective gas, mixing the sintering product, a lithium source, a phosphorus source and a carbon source B, and performing a calcination treatment to obtain a carbon-coated lithium iron manganese phosphate material.

[0005] By mixing a manganese source, an iron source and an alkali solution and performing a precipitation reaction, with the alkali solution as a precipitating agent, it is helpful to prepare an iron manganese compound with uniform distribution of manganese elements and iron elements, thereby contributing to improving the structural stability. By mixing the iron manganese compound and a carbon source A and successively performing a pre-sintering treatment and a reduction sintering treatment, the pre-sintering treatment is carried out at a relatively low temperature, which is helpful for the carbon source A to be uniformly distributed on the surface of the iron manganese compound to form a preliminary carbon layer. Mixing the reduction sintering product of Step S2, a lithium source, a phosphorus source and a carbon source B and performing a calcination treatment. The addition of the carbon source B provides additional carbon coating, which is helpful for further enhancing the conductivity of the lithium iron manganese phosphate material. The double carbon coating is helpful for further inhibiting the dissolution of metal ions.

[0006] Further, in the above step S2, the mass ratio of the iron-manganese compound to carbon source A is 100:(3-8); and / or, the temperature of the pre-sintering treatment is 350-450 °C; and / or, the heat preservation time of the pre-sintering treatment is 30-60 min; the temperature of the reduction-sintering treatment is 600-650 °C; and / or, the heat preservation time of the reduction-sintering treatment is 1-5 h; preferably, carbon source A is an organic compound; more preferably, the organic compound is selected from any one or more of glucose, polyvinyl alcohol, and starch.

[0007] Controlling the mass ratio of the iron-manganese compound to carbon source A within the above range helps to control the formation of a carbon layer with an appropriate thickness on the surface of the iron-manganese compound, thereby helping to balance the conductivity of the material and the diffusion efficiency of lithium ions. Controlling the temperature and heat preservation time of the pre-sintering treatment within the above range helps to improve the uniformity of the distribution of carbon source A on the surface of the iron-manganese compound. Controlling the temperature and heat preservation time of the reduction-sintering treatment within the above range helps to further improve the stability and structural integrity of carbon coating. Controlling the type of carbon source A within the above range is beneficial to improving the purity and electrochemical performance of the material.

[0008] Further, the above step S2 includes: step S21, in a protective gas, mixing the iron-manganese compound and carbon source A and then performing a pre-sintering treatment to obtain a pre-sintered product; step S22, in a reducing gas, performing a reduction-sintering treatment on the pre-sintered product to obtain a reduction-sintered product; preferably, the reducing gas is a mixed gas of hydrogen and nitrogen, and more preferably, the volume ratio of hydrogen to nitrogen is (1-3):(8-10).

[0009] Performing the pre-sintering treatment in a protective gas helps to reduce the oxidation of the material. Hydrogen, as a reducing agent, can effectively reduce the metal ions in the iron-manganese compound and promote the formation of the MnO and FeO structures. Controlling the volume ratio of hydrogen to nitrogen within the above range helps to improve the efficiency and safety of the reduction-sintering treatment.

[0010] Further, in the above step S3, the molar ratio of lithium element in the lithium source, phosphorus element in the phosphorus source, manganese element in the sintered product, and iron element in the sintered product is (1.02-1.10):1:(0.4-0.7):(0.3-0.6); and / or, the total mass of the lithium source and the phosphorus source and the mass ratio of the carbon source B is 100:(0.5-1.2); and / or, the temperature of the calcination treatment is 600-750 °C; and / or, the time of the calcination treatment is 3-8 h; preferably, the lithium source is selected from any one or more of lithium carbonate, lithium hydroxide, and lithium oxide; and / or, the phosphorus source is selected from any one or more of phosphorus pentoxide, phosphoric acid, and ammonium dihydrogen phosphate; and / or, the carbon source B is a carbon material; preferably, the carbon material is selected from any one or more of carbon nanotubes, carbon black, and graphene.

[0011] Controlling the molar ratios of lithium element in the lithium source, phosphorus element in the phosphorus source, manganese element in the sintered product, and iron element in the sintered product within the above ranges helps to improve the stability and activity of the material during electrochemical cycling. Controlling the ratio of the total mass of the lithium source and the phosphorus source to the mass of carbon source B within the above range helps to form a suitable carbon layer on the surface of the material. Controlling the temperature and holding time of the calcination treatment within the above ranges helps to form a stable lithium iron manganese phosphate phase. Controlling the types of the lithium source and the phosphorus source within the above ranges helps to form a more stable lithium iron manganese phosphate phase. Controlling the type of carbon source B within the above range helps to further improve the electrical conductivity of the carbon-coated lithium iron manganese phosphate material.

[0012] Further, in the above step S1, the stoichiometric ratio of manganese element in the manganese source to iron element in the iron source is (4 - 7):(3 - 6); and / or, the stoichiometric ratio of OH - in the alkali solution is 1 - 2 times the total stoichiometry of manganese element in the manganese source and iron element in the iron source; and / or, the temperature of the precipitation reaction is 20 - 30 °C; and / or, the time of the precipitation reaction is 20 - 28 h; preferably, the manganese source is selected from any one or more of manganese sulfate, manganese chloride, and manganese nitrate; and / or, the iron source is selected from any one or more of iron sulfate, iron chloride, and iron nitrate; and / or, the alkali solution is any one or more of sodium hydroxide solution and / or potassium hydroxide solution.

[0013] Controlling the stoichiometric ratio of manganese element in the manganese source to iron element in the iron source within the above range helps to form a structure with high electrochemical activity. Controlling the stoichiometric ratio of OH - in the alkali solution within the above range helps to improve the purity of the iron manganese compound and the integrity of the structure. Controlling the temperature and time of the precipitation reaction within the above ranges helps to improve the crystallinity and activity of the iron manganese compound. Controlling the types of the manganese source, the iron source, and the alkali solution within the above ranges helps to further improve the efficiency of the precipitation reaction.

[0014] Further, in the above step S1, subjecting the product of the precipitation reaction to solid-liquid separation, washing, drying, and sieving in sequence to obtain the iron manganese compound helps to improve the purity of the iron manganese compound.

[0015] Further, sieving is carried out using a sieve, and the mesh number of the sieve is 30 - 70 meshes; and / or, the drying temperature is 80 - 100 °C; and / or, washing is carried out using a washing liquid, and the washing liquid is selected from any one or more of acetone, ethanol, and water.

[0016] Controlling the mesh number of the sieve within the above range helps to remove larger particles or aggregates generated during the precipitation process. Controlling the drying temperature within the above range helps to obtain loose and easily dispersible powder. Controlling the type of washing liquid within the above range helps to improve the efficiency of removing impurities.

[0017] According to another aspect of the present invention, there is provided a carbon-coated lithium iron manganese phosphate material, which is prepared by the aforementioned preparation method.

[0018] The above carbon-coated lithium iron manganese phosphate material has a high specific capacity and cycle stability.

[0019] According to still another aspect of the present invention, there is provided a positive electrode sheet, including a current collector and a positive electrode active layer, and the positive electrode active layer contains the aforementioned carbon-coated lithium iron manganese phosphate material.

[0020] The above positive electrode sheet has a high specific capacity and cycle stability.

[0021] According to still another aspect of the present invention, there is provided a lithium ion battery, including a positive electrode sheet, an electrolyte and a negative electrode sheet, and the positive electrode sheet is the aforementioned positive electrode sheet.

[0022] The above lithium ion battery has a high specific capacity and cycle stability.

[0023] Applying the technical solution of the present application, in step S1 of the present application, by mixing a manganese source, an iron source and an alkali solution and then carrying out a precipitation reaction, the alkali solution serves as a precipitant, which helps to prepare an iron manganese compound with uniform distribution of manganese and iron elements, thereby helping to improve the structural stability, and the iron manganese compound has high activity, and lithium elements are easy to diffuse into it, and crystals can grow at a lower temperature, which helps to reduce the temperature of subsequent calcination treatment, achieving the benefits of cost reduction and efficiency improvement. In step S2, by mixing the iron manganese compound and carbon source A and then carrying out pre-sintering treatment and reduction sintering treatment in sequence, the pre-sintering treatment is carried out at a lower temperature, which helps carbon source A to be uniformly distributed on the surface of the iron manganese compound to form a preliminary carbon layer. The subsequent reduction sintering treatment is carried out at a higher temperature, which helps to further form a stable carbon-coated structure and at the same time promotes the reduction of metal ions, thereby helping to reduce the dissolution of metal ions and enhance the structural stability of the material. In step S3, under a protective gas, the reduction sintering product of step S2, a lithium source, a phosphorus source and carbon source B are mixed and then calcined. The addition of carbon source B provides additional carbon coating, which helps to further enhance the conductivity of the lithium iron manganese phosphate material. The double carbon coating helps to further inhibit the dissolution of metal ions. Carrying out the calcination treatment under a protective gas helps to form an ordered crystal structure, thereby helping to improve the chemical stability and electrochemical performance of the lithium iron manganese phosphate material. Specific embodiments

[0024] It should be noted that, without conflict, the embodiments in this application and the features in the embodiments can be combined with each other. The present invention will be described in detail below with reference to the embodiments.

[0025] As analyzed in the background art of this application, in the prior art, the specific capacity and cycle stability of the carbon-coated lithium iron manganese phosphate material are relatively low. To solve this problem, this application provides a carbon-coated lithium iron manganese phosphate material, its preparation method, a positive electrode sheet, and a lithium ion battery.

[0026] In a typical embodiment of this application, a preparation method of a carbon-coated lithium iron manganese phosphate material is provided. The preparation method includes the following steps: Step S1, mixing raw materials including a manganese source, an iron source, and an alkali solution and then performing a precipitation reaction to obtain an iron manganese compound; Step S2, mixing the iron manganese compound and a carbon source A and then performing a pre-sintering treatment and a reduction sintering treatment in sequence to obtain a reduction sintering product; Step S3, in a protective gas, mixing the sintering product, a lithium source, a phosphorus source, and a carbon source B and then performing a calcination treatment to obtain the carbon-coated lithium iron manganese phosphate material.

[0027] In Step S1 of this application, by mixing a manganese source, an iron source, and an alkali solution and then performing a precipitation reaction, with the alkali solution as a precipitant, it helps to prepare an iron manganese compound with uniform distribution of manganese and iron elements, thereby helping to improve the structural stability. Moreover, the iron manganese compound has high activity, and lithium elements can easily diffuse into it, and crystals can grow at a relatively low temperature, which helps to reduce the temperature of the subsequent calcination treatment, achieving the benefits of cost reduction and efficiency improvement. In Step S2, by mixing the iron manganese compound and a carbon source A and then performing a pre-sintering treatment and a reduction sintering treatment in sequence, the pre-sintering treatment is carried out at a relatively low temperature, which helps the carbon source A to be uniformly distributed on the surface of the iron manganese compound to form a preliminary carbon layer. Subsequently, the reduction sintering treatment is carried out at a relatively high temperature, which helps to further form a stable carbon-coated structure and promote the reduction of metal ions, and thus helps to reduce the dissolution of metal ions and enhance the structural stability of the material. In Step S3, under a protective gas, the reduction sintering product of Step S2, a lithium source, a phosphorus source, and a carbon source B are mixed and then subjected to a calcination treatment. The addition of the carbon source B provides additional carbon coating, which helps to further enhance the conductivity of the lithium iron manganese phosphate material. The double carbon coating helps to further inhibit the dissolution of metal ions. Carrying out the calcination treatment under a protective gas helps to form an ordered crystal structure, which helps to improve the chemical stability and electrochemical performance of the lithium iron manganese phosphate material.

[0028] It should be noted that the protective gas in this application is selected from any one or more of nitrogen, helium, and argon.

[0029] In an embodiment of the present application, in the above step S2, the mass ratio of the iron-manganese compound to the carbon source A is 100:(3-8); and / or, the temperature of the pre-sintering treatment is 350-450 °C; and / or, the heat preservation time of the pre-sintering treatment is 30-60 min; the temperature of the reduction-sintering treatment is 600-650 °C; and / or, the heat preservation time of the reduction-sintering treatment is 1-5 h; preferably, the carbon source A is an organic compound; further preferably, the organic compound is selected from any one or more of glucose, polyvinyl alcohol, and starch.

[0030] Controlling the mass ratio of the iron-manganese compound to the carbon source A within the above range helps to control the formation of a carbon layer with an appropriate thickness on the surface of the iron-manganese compound, thereby helping to balance the conductivity of the material and the diffusion efficiency of lithium ions, and further helping to improve the overall performance of the battery. Controlling the temperature and heat preservation time of the pre-sintering treatment within the above range helps to improve the uniformity of the distribution of the carbon source A on the surface of the iron-manganese compound and form a preliminary carbon layer. Controlling the temperature and heat preservation time of the reduction-sintering treatment within the above range helps to further improve the stability and structural integrity of the carbon coating. Controlling the type of the carbon source A within the above range, these substances can be converted into an efficient carbon coating layer during the heat treatment process, and at the same time they have good dispersibility and can be evenly distributed on the surface of the iron-manganese compound. In addition, these carbon sources produce less ash and impurities during pyrolysis, which is beneficial to improving the purity and electrochemical performance of the material.

[0031] In an embodiment of the present application, in the above step S1, the manganese source and the iron source are added to the reactor, and a protective gas is introduced for 2-4 h. Then an alkali solution is added, and at the same time, the protective gas is replaced with air for precipitation reaction to obtain the iron-manganese compound, which helps to further improve the purity of the iron-manganese compound and the efficiency of the precipitation reaction.

[0032] In an embodiment of the present application, the above carbon source A is a combination of glucose, polyvinyl alcohol, and starch, and the mass ratio of glucose, polyvinyl alcohol, and starch is (2-5):(0.5-2):(0.2-0.5).

[0033] Glucose, polyvinyl alcohol, and starch can form carbon layers with different properties during pyrolysis. Glucose can decompose to form a carbon layer at a relatively low temperature, which helps to quickly form an initial carbon coating on the material surface and improve conductivity. Polyvinyl alcohol decomposes at a relatively high temperature and can form a denser carbon layer to enhance the structural stability of the carbon coating. Starch can produce a porous carbon structure during decomposition, which helps the rapid diffusion of lithium ions. Controlling the mass ratio of glucose, polyvinyl alcohol, and starch within the above range helps to form a more uniform and stable carbon coating layer, thereby helping to improve the overall performance of the carbon-coated lithium iron phosphate manganese material.

[0034] In an embodiment of the present application, the above-mentioned step S2 includes: step S21, in a protective gas, mixing an iron-manganese compound and a carbon source A and then performing a pre-sintering treatment to obtain a pre-sintered product; step S22, in a reducing gas, performing a reduction-sintering treatment on the pre-sintered product to obtain a reduction-sintered product; preferably, the reducing gas is a mixed gas of hydrogen and nitrogen, and further preferably, the volume ratio of hydrogen to nitrogen is (1-3):(8-10).

[0035] Performing the pre-sintering treatment in a protective gas helps to reduce the oxidation of the material due to reaction with oxygen during heating. Hydrogen, as a reducing agent, can effectively reduce metal ions in the iron-manganese compound at high temperatures, promote the formation of the MnO and FeO structures, and controlling the volume ratio of hydrogen to nitrogen within the above range helps to improve the efficiency and safety of the reduction-sintering treatment.

[0036] In an embodiment of the present application, in the above-mentioned step S3, the molar ratio of lithium element in the lithium source, phosphorus element in the phosphorus source, manganese element in the sintered product, and iron element in the sintered product is (1.02-1.10):1:(0.4-0.7):(0.3-0.6); and / or, the total mass ratio of the lithium source and the phosphorus source to the mass of the carbon source B is 100:(0.5-1.2), specifically, it can be 100:0.5, 100:0.6, 100:0.7, 100:0.8, 100:0.9, 100:1.0, 100:1.1, 100:1.2, and the range values between any two ratios; and / or, the temperature of the calcination treatment is 600-750°C; and / or, the time of the calcination treatment is 3-8 h; preferably, the lithium source is selected from any one or more of lithium carbonate, lithium hydroxide, and lithium oxide; and / or, the phosphorus source is selected from any one or more of phosphorus pentoxide, phosphoric acid, and ammonium dihydrogen phosphate; and / or, the carbon source B is a carbon material; preferably, the carbon material is selected from any one or more of carbon nanotubes, carbon black, and graphene.

[0037] Controlling the molar ratios of lithium element in the lithium source, phosphorus element in the phosphorus source, manganese element in the sintered product, and iron element in the sintered product within the above ranges helps to optimize the chemical composition of the lithium iron manganese phosphate material and improve the stability and activity of the material during electrochemical cycling. Controlling the ratio of the total mass of the lithium source and the phosphorus source to the mass of the carbon source B within the above ranges helps to form a suitable carbon layer on the surface of the material, thereby helping to improve the conductivity of the material without sacrificing the energy density of the material. Controlling the temperature and holding time of the calcination treatment within the above ranges helps to form a stable lithium iron manganese phosphate phase, which in turn helps to improve the cycle stability and electrochemical performance of the carbon-coated lithium iron manganese phosphate material, and helps to improve the production efficiency of lithium iron manganese phosphate and reduce energy consumption. Controlling the types of the lithium source and the phosphorus source within the above ranges helps to enhance the synergistic effect among the components, thereby helping to form a more stable lithium iron manganese phosphate phase. Controlling the type of the carbon source B within the above ranges helps to further improve the conductivity of the carbon-coated lithium iron manganese phosphate material.

[0038] In an embodiment of the present application, in the above step S1, the stoichiometric ratio of manganese element in the manganese source to iron element in the iron source is (4-7):(3-6); and / or, the stoichiometric ratio of OH - in the alkali solution is 1-2 times the total stoichiometry of manganese element in the manganese source and iron element in the iron source; and / or, the temperature of the precipitation reaction is 20-30°C; and / or, the time of the precipitation reaction is 20-28 h; preferably, the manganese source is selected from any one or more of manganese sulfate, manganese chloride, and manganese nitrate; and / or, the iron source is selected from any one or more of iron sulfate, iron chloride, and iron nitrate; and / or, the alkali solution is any one or more of sodium hydroxide solution and / or potassium hydroxide solution.

[0039] Controlling the stoichiometric ratio of manganese element in the manganese source to iron element in the iron source within the above ranges helps to form a structure with high electrochemical activity and at the same time promotes the structural stability of the material after multiple charge-discharge cycles. Controlling the stoichiometric ratio of OH - in the alkali solution within the above ranges helps to improve the purity and structural integrity of the iron-manganese compound. Controlling the temperature and time of the precipitation reaction within the above ranges helps to improve the crystallinity and activity of the iron-manganese compound. Controlling the types of the manganese source, the iron source, and the alkali solution within the above ranges helps to further improve the efficiency of the precipitation reaction.

[0040] In order to further improve the purity of the iron-manganese compound, in an embodiment of the present application, it is preferred that in the above step S1, the product of the precipitation reaction is successively subjected to solid-liquid separation, washing, drying, and sieving to obtain the iron-manganese compound.

[0041] In an embodiment of the present application, sieving is carried out using a sieve, and the mesh number of the sieve is 30 to 70 meshes; and / or, the drying temperature is 80 to 100 °C; and / or, washing is carried out using a washing liquid, and the washing liquid is selected from any one or more of acetone, ethanol, and water.

[0042] Controlling the mesh number of the sieve within the above range helps to remove larger particles or aggregates generated during the precipitation process, helps to refine the particle size of the iron-manganese compound, and the small particle size helps to increase the specific surface area of the iron-manganese compound, which is beneficial for the subsequent diffusion of lithium elements and carbon coating. Controlling the drying temperature within the above range helps to obtain loose and easily dispersible powder, which is convenient for subsequent mixing and carbon coating treatment, and at the same time reduces the side reactions and increased energy consumption that may be brought about by high-temperature drying. Controlling the type of washing liquid within the above range helps to improve the efficiency of removing impurities.

[0043] In another typical embodiment of the present application, a carbon-coated lithium iron manganese phosphate material is provided, and the carbon-coated lithium iron manganese phosphate material is prepared by the aforementioned preparation method.

[0044] The carbon-coated lithium iron manganese phosphate material prepared by the preparation method of the present application has a double-layer carbon coating structure. On the one hand, it helps to inhibit the dissolution of metal ions, and on the other hand, it helps to improve the conductivity of the carbon-coated lithium iron manganese phosphate material, thereby helping to improve the specific capacity and cycle stability of the carbon-coated lithium iron manganese phosphate material.

[0045] In yet another typical embodiment of the present application, a positive electrode sheet is provided, which includes a current collector and a positive electrode active layer, and the positive electrode active layer contains the aforementioned carbon-coated lithium iron manganese phosphate material.

[0046] Since the above positive electrode sheet contains the carbon-coated lithium iron manganese phosphate material of the present application, the positive electrode sheet has a high specific capacity and cycle stability.

[0047] In yet another typical embodiment of the present application, a lithium ion battery is provided, which includes a positive electrode sheet, an electrolyte, and a negative electrode sheet, and the positive electrode sheet is the aforementioned positive electrode sheet.

[0048] Since the positive electrode sheet of the above lithium ion battery contains the carbon-coated lithium iron manganese phosphate material of the present application, the lithium ion battery has a high specific capacity and cycle stability.

[0049] The beneficial effects of the present application will be further described below in conjunction with embodiments.

[0050] Example 1

[0051] S1. Manganese sulfate and iron sulfate are added to a five-necked flask according to the stoichiometric ratio of manganese element: iron element = 6:4. After purging with nitrogen for 4 h, a sodium hydroxide solution is added, and the OH in the sodium hydroxide solution- The stoichiometric ratio is 1.2 times the sum of manganese and iron elements. At the same time, nitrogen is replaced with air. After a precipitation reaction at 25 °C for 22 h, suction filtration is carried out, and after washing with acetone, it is dried at 85 °C in a nitrogen atmosphere. The obtained sample is passed through a 40-mesh sieve to obtain an iron-manganese compound;

[0052] S2. Mix the iron-manganese compound with glucose. The addition amount of glucose is 5% of the mass of the iron-manganese compound. After pre-sintering treatment at 350 °C for 30 min and then cooling to 25 °C, H 2 / N 2 =1:10 mixed gas is introduced, and the temperature is raised to 600 °C for reduction sintering treatment, and kept warm for 3 h to obtain a reduction-sintered product;

[0053] S3. Mix the reduction-sintered product with lithium carbonate, phosphorus pentoxide and carbon nanotubes to obtain a mixture. The molar ratio of Li:P:Mn:Fe in the mixture is 1.02:1:0.6:0.4. The addition amount of carbon nanotubes is 0.8% of the total mass of lithium carbonate and phosphorus pentoxide. The mixture is calcined at 700 °C in high-purity nitrogen for 6 h to obtain a carbon-coated lithium iron manganese phosphate material.

[0054] Example 2

[0055] The difference from Example 1 is that in S1, manganese sulfate and iron sulfate are added to a five-necked flask according to the stoichiometric ratio of manganese element:iron element = 7:3. After introducing nitrogen for 3 h, sodium hydroxide solution is added. The OH - in the sodium hydroxide solution has a stoichiometric ratio of 1.5 times the sum of manganese and iron elements. At the same time, nitrogen is replaced with air. After a precipitation reaction at 25 °C for 26 h, suction filtration is carried out, and after washing with acetone, it is dried at 90 °C in a nitrogen atmosphere. The obtained sample is passed through a 70-mesh sieve to obtain an iron-manganese compound;

[0056] S2. Mix the iron-manganese compound with polyvinyl alcohol. The addition amount of polyvinyl alcohol is 8% of the mass of the iron-manganese compound. After pre-sintering treatment at 350 °C for 30 min and then cooling to 25 °C, H 2 / N 2 =2:9 mixed gas is introduced, and the temperature is raised to 600 °C for reduction sintering treatment, and kept warm for 3 h to obtain a reduction-sintered product;

[0057] S3. Mix the reduction-sintered product with lithium hydroxide, phosphoric acid and carbon black to obtain a mixture. The molar ratio of Li:P:Mn:Fe in the mixture is 1.05:1:0.7:0.3. The addition amount of carbon black is 1.0% of the total mass of lithium hydroxide and phosphoric acid. The mixture is calcined at 650 °C in high-purity argon for 8 h to obtain a carbon-coated lithium iron manganese phosphate material.

[0058] Example 3

[0059] The difference from Example 1 is as follows: S1. Manganese sulfate and iron sulfate are added into a five-necked flask according to the stoichiometric ratio of manganese element: iron element = 5:5. After purging with nitrogen for 4 h, sodium hydroxide solution is added. In the sodium hydroxide solution, the stoichiometric ratio of OH - is 1.8 times the sum of manganese element and iron element. Meanwhile, nitrogen is replaced with air. After a precipitation reaction at 25 °C for 25 h, suction filtration is carried out, followed by washing with ethanol, and then drying at 100 °C under a nitrogen atmosphere. The obtained sample is passed through a 50-mesh sieve to obtain an iron-manganese compound;

[0060] S2. The iron-manganese compound is mixed with starch. The addition amount of starch is 7% of the mass of the iron-manganese compound. After pre-sintering treatment at 350 °C for 30 min and then cooling to 25 °C, a mixed gas of H 2 / N 2 = 3:8 is introduced, and the temperature is raised to 600 °C for reduction sintering treatment, with heat preservation for 5 h, to obtain a reduction sintered product;

[0061] S3. The reduction sintered product is mixed with lithium oxide, ammonium dihydrogen phosphate and graphene to obtain a mixture. In the mixture, the molar ratio of Li:P:Mn:Fe is 1.10:1:0.5:0.5. The addition amount of graphene is 0.9% of the total mass of lithium oxide and ammonium dihydrogen phosphate. The mixture is calcined at 620 °C in high-purity argon for 8 h to obtain a carbon-coated lithium iron manganese phosphate material.

[0062] Example 4

[0063] The difference from Example 1 is as follows: S1. Manganese sulfate and iron sulfate are added into a five-necked flask according to the stoichiometric ratio of manganese element: iron element = 5:5. After purging with nitrogen for 3 h, sodium hydroxide solution is added. In the sodium hydroxide solution, the stoichiometric ratio of OH - is 1.2 times the sum of manganese element and iron element. Meanwhile, nitrogen is replaced with air. After a precipitation reaction at 25 °C for 28 h, suction filtration is carried out, followed by washing with acetone, and then drying at 95 °C under a nitrogen atmosphere. The obtained sample is passed through a 30-mesh sieve to obtain an iron-manganese compound;

[0064] S2. The iron-manganese compound is mixed with glucose, polyvinyl alcohol and starch. The mass ratio of glucose, polyvinyl alcohol and starch is 2:1:0.2. The total addition amount of glucose, polyvinyl alcohol and starch is 3% of the mass of the iron-manganese compound. After pre-sintering treatment at 350 °C for 30 min and then cooling to 25 °C, a mixed gas of H 2 / N 2 = 1:10 is introduced, and the temperature is raised to 600 °C for reduction sintering treatment, with heat preservation for 3 h, to obtain a reduction sintered product;

[0065] S3. Mix the reduced sintered product with lithium carbonate, ammonium dihydrogen phosphate, and carbon nanotubes to obtain a mixture. The molar ratio of Li:P:Mn:Fe in the mixture is 1.08:1:0.5:0.5, and the addition amount of carbon nanotubes is 1.2% of the total mass of lithium carbonate and ammonium dihydrogen phosphate. Calcinate the mixture in high-purity nitrogen at 750 °C for 4 h to obtain the carbon-coated lithium iron manganese phosphate material.

[0066] Example 5

[0067] The difference from Example 1 is that the addition amount of glucose is 3% of the mass of the iron-manganese compound, and finally the carbon-coated lithium iron manganese phosphate material is obtained.

[0068] Example 6

[0069] The difference from Example 1 is that the addition amount of glucose is 8% of the mass of the iron-manganese compound, and finally the carbon-coated lithium iron manganese phosphate material is obtained.

[0070] Example 7

[0071] The difference from Example 1 is that the addition amount of glucose is 10% of the mass of the iron-manganese compound, and finally the carbon-coated lithium iron manganese phosphate material is obtained.

[0072] Example 8

[0073] The difference from Example 1 is that the temperature of the pre-sintering treatment is 450 °C, the heat preservation time of the pre-sintering treatment is 60 min, the temperature of the reduction sintering treatment is 650 °C, and the heat preservation time of the reduction sintering treatment is 5 h. Finally, the carbon-coated lithium iron manganese phosphate material is obtained.

[0074] Example 9

[0075] The difference from Example 1 is that the temperature of the pre-sintering treatment is 500 °C, the heat preservation time of the pre-sintering treatment is 90 min, the temperature of the reduction sintering treatment is 700 °C, and the heat preservation time of the reduction sintering treatment is 0.5 h. Finally, the carbon-coated lithium iron manganese phosphate material is obtained.

[0076] Example 10

[0077] The difference from Example 1 is that the mass ratio of glucose, polyvinyl alcohol, and starch is 5:05:0.5, and finally the carbon-coated lithium iron manganese phosphate material is obtained.

[0078] Example 11

[0079] The difference from Example 1 is that the addition amount of carbon nanotubes is 0.5% of the total mass of lithium carbonate and phosphorus pentoxide, and finally the carbon-coated lithium iron manganese phosphate material is obtained.

[0080] Example 12

[0081] The difference from Example 1 is that the addition amount of carbon nanotubes is 1.2% of the total mass of lithium carbonate and phosphorus pentoxide, and finally a carbon-coated lithium iron manganese phosphate material is obtained.

[0082] Example 13

[0083] The difference from Example 1 is that the addition amount of carbon nanotubes is 1.5% of the total mass of lithium carbonate and phosphorus pentoxide, and finally a carbon-coated lithium iron manganese phosphate material is obtained.

[0084] Example 14

[0085] The difference from Example 1 is that the temperature of the calcination treatment is 600 °C and the time of the calcination treatment is 8 h, and finally a carbon-coated lithium iron manganese phosphate material is obtained.

[0086] Example 15

[0087] The difference from Example 1 is that the temperature of the calcination treatment is 750 °C and the time of the calcination treatment is 3 h, and finally a carbon-coated lithium iron manganese phosphate material is obtained.

[0088] Example 16

[0089] The difference from Example 1 is that the temperature of the calcination treatment is 800 °C and the time of the calcination treatment is 2 h, and finally a carbon-coated lithium iron manganese phosphate material is obtained.

[0090] Comparative Example 1

[0091] The difference from Example 1 is that the addition of glucose is cancelled, and finally a carbon-coated lithium iron manganese phosphate material is obtained.

[0092] Comparative Example 2

[0093] The difference from Example 1 is that Step S2 is cancelled, manganese sulfate and iron sulfate are added to the five-necked flask according to the stoichiometric ratio of manganese element:iron element = 6:4, after nitrogen is introduced for 4 h, sodium hydroxide solution is added, wherein the stoichiometric ratio of OH in the sodium hydroxide solution - is 1.2 times the sum of manganese element and iron element, and at the same time nitrogen is replaced by air, after precipitation reaction at 25 °C for 22 h, suction filtration is carried out, washed with acetone, dried at 85 °C under a nitrogen atmosphere, and the obtained sample is sieved through a 40-mesh sieve to obtain an iron manganese compound;

[0094] The iron manganese compound is mixed with lithium carbonate, phosphorus pentoxide and carbon nanotubes to obtain a mixture, the molar ratio of Li:P:Mn:Fe in the mixture is 1.02:1:0.6:0.4, the addition amount of carbon nanotubes is 0.8% of the total mass of lithium carbonate and phosphorus pentoxide, and the mixture is calcined at 700 °C in high-purity nitrogen for 6 h to obtain a carbon-coated lithium iron manganese phosphate material.

[0095] Comparative Example 3

[0096] The difference from Example 1 is that steps S1 and S2 are cancelled, and manganese sulfate, iron sulfate are directly mixed with lithium carbonate, phosphorus pentoxide and carbon nanotubes to obtain a mixture. The molar ratio of Li:P:Mn:Fe in the mixture is 1.02:1:0.6:0.4, and the addition amount of carbon nanotubes is 0.8% of the total mass of lithium carbonate and phosphorus pentoxide. The mixture is calcined at 700 °C in high-purity nitrogen for 6 h to obtain the lithium iron manganese phosphate material coated with carbon.

[0097] Performance Test

[0098] The lithium iron manganese phosphate materials coated with carbon prepared in the examples and comparative examples are used as the positive electrode materials, a lithium sheet is used as the negative electrode material, and lithium hexafluorophosphate is used as the main component of the electrolyte to assemble a button cell. The battery is tested for the charge specific capacity and discharge specific capacity at a rate of 0.2C, the discharge specific capacity at a rate of 1C, the discharge specific capacity at a rate of 2C, and the capacity retention rate after 500 cycles at a rate of 1C within the charge-discharge electrochemical window of 2.5-4.5V. The test results are shown in Table 1.

[0099] Table 1

[0100]

[0101]

[0102] From the above description, it can be seen that the above embodiments of the present invention achieve the following technical effects:

[0103] In step S1 of the present application, by mixing a manganese source, an iron source, and an alkali solution and then carrying out a precipitation reaction, the alkali solution serves as a precipitant, which helps to prepare an iron-manganese compound with uniform distribution of manganese and iron elements, thereby contributing to improving the structural stability. Moreover, the iron-manganese compound has high activity, and lithium elements can easily diffuse into it, and crystals can grow at a relatively low temperature, which helps to reduce the temperature of subsequent calcination treatment, achieving the benefits of cost reduction and efficiency improvement. In step S2, by mixing the iron-manganese compound and carbon source A and then successively carrying out pre-sintering treatment and reduction sintering treatment, the pre-sintering treatment is carried out at a relatively low temperature, which helps carbon source A to be uniformly distributed on the surface of the iron-manganese compound to form a preliminary carbon layer. The subsequent reduction sintering treatment is carried out at a relatively high temperature, which helps to further form a stable carbon-coated structure and at the same time promotes the reduction of metal ions, thereby helping to reduce the dissolution of metal ions and enhance the structural stability of the material. In step S3, under a protective gas, the reduction sintering product of step S2, a lithium source, a phosphorus source, and carbon source B are mixed and then subjected to calcination treatment. The addition of carbon source B provides additional carbon coating, which helps to further enhance the conductivity of the lithium iron phosphate manganese material. The double carbon coating helps to further inhibit the dissolution of metal ions. Carrying out the calcination treatment under a protective gas helps to form an ordered crystal structure, thereby contributing to improving the chemical stability and electrochemical performance of the lithium iron phosphate manganese material.

[0104] The above are only the preferred embodiments of the present invention and are not intended to limit the present invention. For those skilled in the art, the present invention can have various modifications and changes. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A method for preparing a carbon-coated lithium manganese iron phosphate material, characterized in that: The preparation method comprises the following steps: Step S1, mixing raw materials including a manganese source, an iron source and an alkaline solution and performing a precipitation reaction to obtain an iron-manganese compound; Step S2, mixing the iron-manganese compound and the carbon source A, and sequentially performing a pre-sintering treatment and a reduction sintering treatment to obtain a reduction sintered product; Step S3, in a protective gas, mixing the sintered product, a lithium source, a phosphorus source and a carbon source B and then calcining them to obtain the carbon-coated lithium manganese iron phosphate material.

2. The preparation method according to claim 1, characterized in that: In the step S2, the mass ratio of the iron-manganese compound to the carbon source A is 100:(3-8); and / or, the temperature of the pre-sintering treatment is 350-450° C.; and / or, the holding time of the pre-sintering treatment is 30-60 min; the temperature of the reduction sintering treatment is 600-650° C.; and / or, the holding time of the reduction sintering treatment is 1-5 h; Preferably, the carbon source A is an organic compound; further preferably, the organic compound is selected from any one or more of glucose, polyvinyl alcohol and starch.

3. The preparation method according to claim 1 or 2, characterized in that: The step S2 comprises: Step S21, in a protective gas, mixing the iron-manganese compound and the carbon source A and performing the pre-sintering treatment to obtain a pre-sintered product; Step S22, subjecting the pre-sintered product to the reduction sintering treatment in a reducing gas to obtain the reduction sintered product; Preferably, the reducing gas is a mixed gas of hydrogen and nitrogen. More preferably, the volume ratio of the hydrogen and nitrogen is (1-3):(8-10).

4. The preparation method according to any one of claims 1 to 3, characterized in that In the step S3, the molar ratio of the lithium element in the lithium source, the phosphorus element in the phosphorus source, the manganese element in the sintered product and the iron element in the sintered product is (1.02-1.10):1:(0.4-0.7):(0.3-0.6); And / or, the ratio of the total mass of the lithium source and the phosphorus source to the mass of the carbon source B is 100:(0.5-1.2); And / or, the calcination temperature is 600-750°C; and / or, the calcination time is 3-8h; Preferably, the lithium source is selected from any one or more of lithium carbonate, lithium hydroxide and lithium oxide; And / or, the phosphorus source is selected from any one or more of phosphorus pentoxide, phosphoric acid and ammonium dihydrogen phosphate; And / or, the carbon source B is a carbon material; preferably, the carbon material is selected from any one or more of carbon nanotubes, carbon black and graphene.

5. The preparation method according to any one of claims 1 to 4, characterized in that: In the step S1, the stoichiometric ratio of manganese in the manganese source to iron in the iron source is (4-7):(3-6); and / or, OH in the alkaline solution - The stoichiometric ratio is 1 to 2 times the total stoichiometric ratio of the manganese element in the manganese source to the iron element in the iron source; And / or, the temperature of the precipitation reaction is 20-30°C; and / or, the time of the precipitation reaction is 20-28h; Preferably, the manganese source is selected from any one or more of manganese sulfate, manganese chloride and manganese nitrate; And / or, the iron source is selected from any one or more of ferric sulfate, ferric chloride and ferric nitrate; And / or, the alkaline solution is any one or more of a sodium hydroxide solution and / or a potassium hydroxide solution.

6. The preparation method according to any one of claims 1 to 5, characterized in that In the step S1, the product of the precipitation reaction is sequentially subjected to solid-liquid separation, washing, drying and sieving to obtain the iron-manganese compound.

7. The preparation method according to claim 6, characterized in that: The sieving is performed using a sieve with a mesh size of 30 to 70; and / or the drying temperature is 80 to 100° C.; and / or the washing is performed using a washing liquid selected from any one or more of acetone, ethanol and water.

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

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