Method for preparing carbon-coated lithium iron manganese phosphate positive electrode active material and production system

Abstract

The application provides a preparation method and a production system of a carbon-coated lithium manganese iron phosphate positive electrode active material. The preparation method comprises the following steps: under 0.2-1.6 MPa and at 120-200 DEG C, a raw material solution comprising a lithium source, an iron source, a manganese source and a phosphorus source is subjected to ultrasonic reaction to obtain lithium manganese iron phosphate, in the ultrasonic reaction, the ultrasonic power is not less than 3 KW / m 3 ; the lithium manganese iron phosphate is sintered with a carbon source in a protective atmosphere to obtain the carbon-coated lithium manganese iron phosphate positive electrode active material; wherein the solid content of the lithium manganese iron phosphate in the reaction system after the ultrasonic reaction is 40-100 kg / m 3 ; the lithium manganese iron phosphate comprises primary particles and secondary particles formed by the primary particles, and the median particle size of the primary particles is 50-200 nm. Through the preparation method, the primary particles with small particle size can be prepared under the condition of high solid content, and the purpose of cost reduction and efficiency increase is achieved.

Description

Technical Field

[0001] This invention belongs to the field of lithium-ion batteries and relates to a method for preparing a carbon-coated lithium manganese iron phosphate cathode active material, and more particularly to a method and production system for preparing a carbon-coated lithium manganese iron phosphate cathode active material. Background Technology

[0002] Lithium manganese iron phosphate (LMFP) is a new type of positive electrode active material obtained by doping manganese with lithium iron phosphate. It is an upgraded version of lithium iron phosphate. Its crystal structure is similar to that of lithium iron phosphate, and it has stable chemical properties and excellent safety performance. The theoretical battery energy density is 15-20% higher than that of lithium iron phosphate, and it has good market prospects.

[0003] Currently, the main production methods for lithium manganese iron phosphate include solid-phase method, liquid-phase method, and solid-liquid two-phase method. Among them, the liquid-phase method has a significant advantage in achieving uniformity of lithium manganese iron phosphate materials. However, in the industrial production process of the liquid-phase method, the concentration of the precursor mixed solution needs to be kept within a low range to obtain lithium manganese iron phosphate cathode active material with good electrochemical performance. When the concentration of the mixed solution is too high, it may lead to an excessively high solid content of lithium manganese iron phosphate generated in the reaction system, resulting in excessively large primary particle size or agglomeration of lithium manganese iron phosphate. This is not only detrimental to improving the electrochemical performance of lithium manganese iron phosphate cathode active material, but also results in low production efficiency.

[0004] Therefore, there is an urgent need to develop a method for preparing lithium manganese iron phosphate cathode active materials that can produce primary particles with smaller particle sizes even with a high solid content of lithium manganese iron phosphate, thereby achieving the goal of reducing costs and increasing efficiency. Summary of the Invention

[0005] To address the aforementioned shortcomings, this invention provides a method for preparing carbon-coated lithium manganese iron phosphate cathode active material, which can achieve the preparation of small-particle-size primary lithium manganese iron phosphate particles under conditions of high lithium manganese iron phosphate solid content, thereby achieving the goal of cost reduction and efficiency improvement.

[0006] This invention provides a production system for carbon-coated lithium manganese iron phosphate cathode active material, applicable to the above-mentioned preparation method of carbon-coated lithium manganese iron phosphate cathode active material. It includes an ultrasonic mixing solvothermal reaction crystallization device and a sintering device, which can produce primary lithium manganese iron phosphate particles with smaller particle size under high solid content of lithium manganese iron phosphate, thereby improving production efficiency and reducing production costs.

[0007] This invention provides a method for preparing a carbon-coated lithium manganese iron phosphate cathode active material, comprising the following steps:

[0008] (1) Under 0.2–1.6 MPa and at 120–200 °C, a raw material solution including lithium, iron, manganese, and phosphorus sources is subjected to an ultrasonic reaction to obtain lithium manganese iron phosphate; wherein the ultrasonic power in the ultrasonic reaction is not less than 3 kW / m 3 ;

[0009] (2) The lithium manganese iron phosphate and a carbon source are sintered under a protective atmosphere to obtain the carbon-coated lithium manganese iron phosphate positive electrode active material.

[0010] The solid content of lithium manganese iron phosphate in the reaction system after the ultrasonic reaction is completed is 40-100 kg / m³. 3 ;

[0011] The lithium manganese iron phosphate comprises primary particles and secondary particles formed from the primary particles, wherein the median particle size of the primary particles is 50–200 nm.

[0012] Furthermore, the mass ratio of the lithium manganese iron phosphate material to the carbon source is 0.85:0.15 to 0.95:0.05.

[0013] Furthermore, in step (1), the ratio of the molar amount of lithium ions, the total molar amount of iron ions and manganese ions, and the molar amount of phosphate ions in the raw material solution is (2.7~3.2):1:1;

[0014] And / or, the conditions for the ultrasonic reaction are: a heating rate of 0.5 to 20 °C / min and a holding time of 0.5 to 24 h.

[0015] Furthermore, step (1) also includes, after the ultrasonic reaction is completed, solid-liquid separation treatment is performed on the reaction system to obtain crude lithium manganese iron phosphate and separation mother liquor; the crude lithium manganese iron phosphate is washed to obtain lithium manganese iron phosphate and washing mother liquor.

[0016] Furthermore, the washing process includes:

[0017] The crude lithium manganese iron phosphate product is first washed with a first washing solution including deionized water to obtain a first crude lithium manganese iron phosphate product and a first mother liquor.

[0018] The first crude lithium manganese iron phosphate product is washed a second time using a second washing solution including deionized water to obtain a second crude lithium manganese iron phosphate product and a second mother liquor.

[0019] The crude lithium manganese phosphate product was subjected to a third washing with a third washing solution including deionized water to obtain lithium manganese phosphate and a third mother liquor.

[0020] Furthermore, the washing process also includes:

[0021] The solids in the separated mother liquor and the first mother liquor are introduced into the first crude lithium manganese iron phosphate product for recycling to participate in the second washing; and / or...

[0022] The second mother liquor is introduced into the first washing solution and circulated to participate in the first washing process; and / or,

[0023] The third mother liquor is introduced into the second washing liquid and circulated to participate in the second washing process.

[0024] Furthermore, step (2) also includes mixing the lithium manganese iron phosphate, the carbon source, and deionized water to obtain a slurry; grinding the slurry to obtain a crystal slurry with a median particle size D50 of 50-200 nm and D97 < 300 nm; granulating the crystal slurry to obtain lithium manganese iron phosphate mixed carbon source particles with a median particle size D50 of 1-20 μm and a residual water content of < 1%; and sintering the lithium manganese iron phosphate mixed carbon source particles under a protective atmosphere to obtain the carbon-coated lithium manganese iron phosphate positive electrode active material.

[0025] Furthermore, the sintering conditions are as follows: sintering temperature is 650–750℃, heating rate is 1–5℃ / min, and holding time is 1.0–6.0h.

[0026] Furthermore, the lithium source is lithium hydroxide monohydrate, the iron source is ferrous sulfate heptahydrate, the manganese source is manganese sulfate monohydrate, and the phosphorus source is phosphoric acid;

[0027] The separated mother liquor and the first mother liquor are subjected to a first liquid phase recovery treatment to obtain a first recovered liquid phase. The first recovered liquid phase is mixed with barium hydroxide octahydrate solution for reaction. The reaction solution is subjected to a second liquid phase recovery treatment to obtain a second recovered liquid phase. The second recovered liquid phase is concentrated and dried to obtain recovered lithium hydroxide monohydrate. The recovered lithium hydroxide is used as the lithium source.

[0028] The molar ratio of barium ions in the barium hydroxide octahydrate solution to sulfate ions in the first recovered liquid phase is 1:1 to 1.01, and the reaction temperature is 15 to 90°C.

[0029] The present invention also provides a production system for carbon-coated lithium manganese iron phosphate cathode active material, applicable to the preparation method of carbon-coated lithium manganese iron phosphate cathode active material described in any of the above claims, including an ultrasonic mixing solvothermal reaction crystallization device and a sintering device;

[0030] The ultrasonic mixed solvothermal reaction crystallization device is used to prepare the lithium manganese iron phosphate, and the sintering device is used to sinter the lithium manganese iron phosphate and the carbon source.

[0031] The output port of the ultrasonic mixed solvothermal reaction crystallization device is connected to the input port of the sintering device.

[0032] The method for preparing carbon-coated lithium manganese iron phosphate cathode active material of the present invention involves carrying out a crystallization reaction of a raw material solution including a lithium source, an iron source, a manganese source, and a phosphorus source under conditions of 0.2–1.6 MPa pressure and 120–200°C temperature, while simultaneously applying a power of not less than 3 kW / m during the reaction process. 3 Ultrasonic waves can not only increase the reaction rate, but also effectively reduce the risk of agglomeration during the ripening stage at the end of lithium manganese iron phosphate crystallization, and reduce the particle size of primary particles generated during the reaction, resulting in lithium manganese iron phosphate with a solid content of 40–100 kg / m³. 3 The reaction system has a median particle size of 50-200 nm for primary lithium manganese iron phosphate particles, achieving the goal of cost reduction and efficiency improvement. Attached Figure Description

[0033] Figure 1 The image shows the XRD pattern of lithium manganese iron phosphate in Example 1 of this invention.

[0034] Figure 2 The charge-discharge curves of the coin cell with carbon-coated lithium manganese iron phosphate cathode active material in Example 1 of the present invention are shown. Detailed Implementation

[0035] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions in the embodiments of this invention will be clearly and completely described below in conjunction with the embodiments of this invention. Obviously, the described embodiments are only some embodiments of this invention, not all embodiments. Based on the embodiments of this invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this invention.

[0036] The first aspect of this invention provides a method for preparing a carbon-coated lithium manganese iron phosphate cathode active material, comprising the following steps:

[0037] (1) Under 0.2–1.6 MPa and at 120–200 °C, a raw material solution including lithium, iron, manganese, and phosphorus sources is subjected to an ultrasonic reaction to obtain lithium manganese iron phosphate; during the ultrasonic reaction, the ultrasonic power is not less than 3 kW / m 3 ;

[0038] (2) Lithium manganese iron phosphate and carbon source are sintered under a protective atmosphere to obtain carbon-coated lithium manganese iron phosphate positive electrode active material.

[0039] The solid content of lithium manganese iron phosphate in the reaction system after the ultrasonic reaction is completed is 40-100 kg / m³. 3 ;

[0040] Lithium manganese iron phosphate includes primary particles and secondary particles formed from primary particles. The median particle size of the primary particles is 50–200 nm.

[0041] Specifically, in step (1), under nitrogen protection, the temperature of the raw material solution, including lithium, iron, manganese, and phosphorus sources, is raised to 120–200°C at a certain heating rate, and the pressure of the reaction system is 0.2–1.6 MPa, with a pressure not less than 3 KW / m 3 The ultrasonic power was used to conduct an ultrasonic reaction to obtain lithium manganese iron phosphate. The solid content of lithium manganese iron phosphate in the reaction system after the ultrasonic reaction was completed was 40–100 kg / m³. 3 Lithium manganese iron phosphate comprises primary particles and secondary particles formed from the primary particles, wherein the median particle size of the primary particles is 50–200 nm.

[0042] In this invention, the pressure of 0.2 to 1.6 MPa refers to an absolute pressure of 0.2 to 1.6 MPa.

[0043] In this invention, the lithium source refers to a raw material providing lithium, the iron source refers to a raw material providing iron, the manganese source refers to a raw material providing manganese, and the phosphorus source refers to a raw material providing phosphorus. Any raw material containing the target element (lithium, iron, manganese, or phosphorus) is considered within the scope of this invention, and one target element can be introduced into the reaction system through one or more raw materials. For example, the lithium source includes at least one of lithium hydroxide monohydrate, lithium carbonate, lithium dihydrogen phosphate, and lithium oxalate; the iron source includes at least one of ferrous sulfate heptahydrate and ferrous chloride tetrahydrate; the manganese source includes at least one of manganese sulfate monohydrate and manganese chloride tetrahydrate; and the phosphorus source includes at least one of phosphoric acid, lithium dihydrogen phosphate, sodium dihydrogen phosphate, and potassium dihydrogen phosphate. It should be explained that when the raw material simultaneously includes two or more of the target elements, the raw material can be understood as a source of two target elements. For example, when the raw material is lithium dihydrogen phosphate, it simultaneously serves as both a lithium source and a phosphorus source.

[0044] This invention does not limit the specific sources of lithium, iron, manganese, and phosphorus sources; for example, they can be obtained through commercially available or conventional preparation methods.

[0045] This invention does not specifically limit the preparation method of the raw material solutions for lithium, iron, manganese, and phosphorus sources. Exemplarily, a lithium source solution can be prepared by mixing deionized water, an organic solvent, and a lithium source; an iron source solution can be prepared by mixing deionized water and an iron source; a manganese source solution can be prepared by mixing a manganese source and deionized water; and a phosphorus source solution can be prepared by mixing a phosphorus source and an organic solvent. In one embodiment, the raw material solution can be obtained by directly mixing the aforementioned lithium, iron, manganese, and phosphorus source solutions. In another embodiment... Alternatively, iron and manganese sources can be mixed with deionized water to prepare a mixed solution of iron and manganese sources. First, lithium source solution and phosphorus source solution are mixed to obtain crystal slurry. Then, the mixed solution of iron and manganese sources is mixed with the crystal slurry to obtain raw material solution. Preferably, the mixed solution of iron and manganese sources may also include an antioxidant to reduce ferric ions or ferric manganese ions and prevent ferrous ions and ferrous manganese ions from being oxidized. Preferably, the deionized water is deoxygenated deionized water to avoid residual oxygen in the deionized water oxidizing ferrous ions or ferrous manganese ions.

[0046] Furthermore, ultrasonic dispersion can be used to make the above solutions more uniformly dispersed during the preparation of lithium source solution, iron source solution, manganese source solution, phosphorus source solution, mixed solution of iron source and manganese source, and raw material solution.

[0047] Furthermore, insoluble impurities in lithium source solution, iron source solution, manganese source solution, phosphorus source solution, and mixed solution of iron and manganese source can be removed by filtration to avoid affecting the electrochemical performance of carbon-coated lithium manganese iron phosphate cathode active material. For example, at least one of ceramic precision filter, bag precision filter, and precision membrane filter can be used for filtration.

[0048] The present invention does not specifically limit the mixing method. For example, the mixing can be carried out by at least one of the following: tubular static mixer, orifice plate mixer, ultrasonic mixer, dynamic mixer, Venturi jet mixer, and colloid mill mixer. It is sufficient to ensure that the lithium source, iron source, manganese source and phosphorus source are fully dissolved and dispersed, and that the lithium source solution, iron source solution, manganese source solution and phosphorus source solution are fully mixed.

[0049] This invention does not impose any special limitations on the types of organic solvents and antioxidants. For example, organic solvents include at least one of ethylene glycol, diethylene glycol, glycerol, and polyethylene glycol; antioxidants include at least one of ascorbic acid, citric acid, oxalic acid, formic acid, acetic acid, and tartaric acid.

[0050] In step (2), the above-mentioned lithium manganese iron phosphate and carbon source are sintered under a protective atmosphere to obtain a sintered product. The sintered product is then pulverized to obtain a carbon-coated lithium manganese iron phosphate positive electrode active material.

[0051] Preferably, the median particle size of the pulverized carbon-coated lithium manganese iron phosphate cathode active material is 50–200 nm, and D97 < 400 nm. In this invention, D50 refers to the particle size value corresponding to a cumulative distribution percentage of 50% from smallest to largest in the particle size distribution, and D97 refers to the particle size value corresponding to a cumulative distribution percentage of 97% from smallest to largest in the particle size distribution.

[0052] The present invention does not specifically limit the pulverization method. For example, it can be pulverized by at least one of ceramic flat air jet mill, fluidized bed air jet mill, and vertical annular jet mill.

[0053] The method for preparing carbon-coated lithium manganese iron phosphate cathode active material of the present invention involves subjecting a raw material solution comprising a lithium source, an iron source, a manganese source, and a phosphorus source to an ultrasonic reaction under conditions of an absolute pressure of 0.2–1.6 MPa and a reaction temperature of 120–200°C, while controlling the ultrasonic power to be no less than 3 kW / m. 3 This process allows the raw material solution to have a high reaction rate during the heating stage of the crystallization reaction. Furthermore, the ultrasonic action promotes collisions between liquid molecules in the raw material solution, generating significant shear force. This helps to break larger lithium manganese iron phosphate particles into smaller ones, achieving the refinement of primary lithium manganese iron phosphate particles and reducing their median particle size. Simultaneously, the isothermal reaction stage prevents the primary lithium manganese iron phosphate particles from agglomerating during the maturation stage at the end of crystallization, thus avoiding particle enlargement. This results in a lithium manganese iron phosphate solid content of 40–100 kg / m³. 3 The reaction system has a median particle size of 50-200 nm for primary lithium manganese iron phosphate particles, which achieves the goal of reducing costs and improving production efficiency.

[0054] In one specific embodiment, in step (1), the ratio of the molar amount of lithium ions, the total molar amount of iron ions and manganese ions, and the molar amount of phosphate ions in the raw material solution is (2.7~3.2):1:1. Within this range, not only can smaller-sized primary particles of lithium manganese iron phosphate be prepared, but the carbon-coated lithium manganese iron phosphate cathode active material obtained by sintering the lithium manganese iron phosphate can also have higher electrochemical performance.

[0055] In one specific embodiment, in step (1), the conditions for the ultrasonic reaction are: a heating rate of 0.5–20 °C / min and a holding time of 0.5–24 h. Within this range, the particle size of the primary particles of lithium manganese iron phosphate can be further reduced, and the crystallization process can be made more complete, reducing internal defects in the crystals, thereby improving the electrochemical performance of the positive electrode active material.

[0056] In one specific embodiment, the mass ratio of lithium manganese iron phosphate to carbon source is 0.85:0.15 to 0.95:0.05. Within this range, a uniform carbon coating layer can be formed on the surface of lithium manganese iron phosphate, increasing electron conduction channels and improving the conductivity of the positive electrode active material, thereby effectively improving the rate performance of the battery.

[0057] In one specific embodiment, step (1) further includes, after the ultrasonic reaction is completed, performing solid-liquid separation on the reaction system to obtain crude lithium manganese iron phosphate and separation mother liquor; and washing the crude lithium manganese iron phosphate to obtain lithium manganese iron phosphate and washing mother liquor. By performing solid-liquid separation on the reaction system and washing the separated crude lithium manganese iron phosphate, impurity ions in the reaction system can be removed, reducing the content of impurity ions in lithium manganese iron phosphate, thereby improving the electrochemical performance of the carbon-coated lithium manganese iron phosphate cathode active material.

[0058] It is understood that the reaction system needs to be cooled before solid-liquid separation. The present invention does not limit the cooling method. For example, it can be cooled by at least one of a wound tube heat exchanger, a plate heat exchanger, and a shell-and-tube heat exchanger. Furthermore, the temperature after cooling is preferably not higher than 60°C.

[0059] The present invention does not limit the method of solid-liquid separation. For example, solid-liquid separation can be performed by at least one of a horizontal sedimentation spiral centrifuge, a disc centrifuge, a tubular centrifuge, and a rotary ceramic membrane filter.

[0060] In one specific embodiment, the washing process includes: first washing the crude lithium manganese iron phosphate product with a first washing solution including deionized water to obtain a first crude lithium manganese iron phosphate product and a first mother liquor; second washing the first crude lithium manganese iron phosphate product with a second washing solution including deionized water to obtain a second crude lithium manganese iron phosphate product and a second mother liquor; and third washing the second crude lithium manganese iron phosphate product with a third washing solution including deionized water to obtain lithium manganese iron phosphate and a third mother liquor.

[0061] Specifically, the crude lithium manganese iron phosphate product is first washed using a first washing solution including deionized water. After washing, solid-liquid separation is performed to obtain a first crude lithium manganese iron phosphate product and a first mother liquor. The first crude lithium manganese iron phosphate product is then second washed using a second washing solution including deionized water. After washing, solid-liquid separation is performed to obtain a second crude lithium manganese iron phosphate product and a second mother liquor. The second crude lithium manganese iron phosphate product is then third washed using a third washing solution including deionized water. After washing, solid-liquid separation is performed to obtain lithium manganese iron phosphate and a third mother liquor. Preferably, the washing temperature during the first, second, and third washing processes is 30–40°C. Preferably, the deionized water is deoxygenated deionized water to avoid oxidation of ferrous or manganese divalent ions.

[0062] Furthermore, ultrasonic and / or agitation methods may be used during the first and / or second and / or third washing processes to make the washing process more thorough.

[0063] The present invention does not impose any special limitation on the solid-liquid separation process. For example, the solid-liquid separation process defined above can be used, which will not be described in detail here.

[0064] The washing method described above can further reduce the concentration of impurity ions in the reaction system, thereby further improving the electrochemical performance of the carbon-coated lithium manganese iron phosphate cathode active material.

[0065] In one specific embodiment, the washing process further includes: introducing the solid phase from the separation mother liquor and the first mother liquor into the first crude lithium manganese iron phosphate product for recycling to participate in the second washing; and / or, introducing the second mother liquor into the first washing liquid for recycling to participate in the first washing; and / or, introducing the third mother liquor into the second washing liquid for recycling to participate in the second washing. Through the above washing process, the amount of washing water used can be reduced, thereby lowering the content of impurity ions in the crude lithium manganese iron phosphate product, reducing costs and water waste; simultaneously, lithium manganese iron phosphate in the separation mother liquor and the first mother liquor can be recovered, avoiding waste and increasing yield.

[0066] It is understood that the solids in the mother liquor and the first mother liquor can be obtained through solid-liquid separation. This invention does not specifically limit the method of solid-liquid separation; for example, solid-liquid separation can be performed using a ceramic membrane filter and / or a rotary ceramic membrane filter.

[0067] In one specific embodiment, step (2) further includes mixing lithium manganese iron phosphate, a carbon source, and deionized water to obtain a slurry; grinding the slurry to obtain a crystalline slurry with a median particle size D50 of 50–200 nm and a D97 < 300 nm; granulating the crystalline slurry to obtain lithium manganese iron phosphate mixed carbon source particles with a median particle size D50 of 1–20 nm and a residual water content < 1%; and sintering the lithium manganese iron phosphate mixed carbon source particles under a protective atmosphere to obtain a carbon-coated lithium manganese iron phosphate positive electrode active material. Preferably, the solid content of the slurry is 0.3–0.5%.

[0068] Preferably, the above-mentioned slurry liquid can be further dispersed by at least one of ultrasonic dispersion, colloid mill dispersion, high-speed shear dispersion, and mechanical dispersion disc to make the slurry liquid more uniformly dispersed.

[0069] This invention does not specifically limit the grinding method; for example, grinding can be performed using a nano-grinding machine.

[0070] The present invention does not specifically limit the granulation method. For example, granulation can be carried out by at least one of ultrasonic spray drying, two-fluid spray dryer, three-fluid spray dryer and high-speed centrifugal spray dryer. Preferably, the inlet air temperature is 200-300°C and the outlet air temperature is 105-115°C.

[0071] Grinding the slurry containing lithium manganese iron phosphate (LMP) makes the LMP particles small and uniform. When the median particle size (D50) after grinding is 50–200 nm and D97 < 300 nm, it helps to improve the electrochemical performance of the carbon-coated LMP cathode active material. Granulating the ground LMP facilitates subsequent carbon coating, making the carbon coating layer more uniform, thereby improving the electrochemical performance of the cathode active material. Furthermore, when the median particle size (D50) of the LMP mixed carbon source particles obtained after granulation is 1–20 μm and the residual water content is < 1%, the cathode active material has even better electrochemical performance.

[0072] The present invention does not specifically limit the type of carbon source. For example, the carbon source may be selected from at least one of glucose, sucrose, polyvinylpyrrolidone (PVP), polyvinylidene fluoride (PVDF), carbon nanotubes, graphene, starch, and cyclodextrin.

[0073] Preferably, the slurry may also include a catalyst, which can improve the degree of carbonization, thereby enhancing the electrochemical performance of the positive electrode active material; it can also lower the carbonization temperature and reduce energy consumption. This invention does not impose specific limitations on the type of catalyst, but may include, for example, nano-silver and / or ferrocene.

[0074] In one specific embodiment, the sintering conditions are: a sintering temperature of 650–750°C, a heating rate of 1–5°C / min, and a holding time of 1.0–6.0 h. Within this range, the final carbon-coated lithium manganese iron phosphate cathode active material can have better crystallinity, reduce the content of impurity phases, and avoid the problems of coarse cathode active material particles and lithium volatilization.

[0075] The present invention does not specifically limit the sintering method. For example, sintering can be carried out by at least one of atmosphere roller furnace, atmosphere sintering rotary furnace, and reducing / inert atmosphere vibrating fluidized bed.

[0076] In one specific embodiment, the lithium source is lithium hydroxide monohydrate, the iron source is ferrous sulfate heptahydrate, the manganese source is manganese sulfate monohydrate, and the phosphorus source is phosphoric acid. The separation mother liquor and the first mother liquor are subjected to a first liquid phase recovery treatment to obtain a first recovered liquid phase. The first recovered liquid phase is mixed with a barium hydroxide octahydrate solution for reaction. The reaction solution is subjected to a second liquid phase recovery treatment to obtain a second recovered liquid phase. The second recovered liquid phase is concentrated and dried to obtain recovered lithium hydroxide monohydrate. The recovered lithium hydroxide is used as the lithium source. The molar ratio of barium ions in the barium hydroxide octahydrate solution to sulfate ions in the first recovered liquid phase is 1:(1~1.01), and the reaction temperature is 15~90℃.

[0077] Specifically, when the lithium source is lithium hydroxide monohydrate, the iron source is ferrous sulfate heptahydrate, the manganese source is manganese sulfate monohydrate, and the phosphorus source is phosphoric acid, deionized water, an organic solvent, and lithium hydroxide monohydrate are mixed to prepare a lithium hydroxide solution. Preferably, the dissolution temperature of lithium hydroxide monohydrate is 30–40°C, the volume ratio of deionized water to organic solvent is (1.5–2.5):1, and the concentration of the lithium hydroxide solution is 50–300 g / L. A mixed solution is prepared by mixing deionized water with ferrous sulfate heptahydrate, manganese sulfate monohydrate, and an antioxidant. Preferably, the dissolution temperature of ferrous sulfate heptahydrate, manganese sulfate monohydrate, and the antioxidant is... The temperature is 30–40°C, the concentration of the mixed solution is 500–1500 g / L, and the molar ratio of ferrous sulfate heptahydrate to manganese sulfate monohydrate is 0.2:0.8–0.4:0.6. An organic solvent is mixed with phosphoric acid to prepare a phosphoric acid solution; preferably, the volume ratio of phosphoric acid to organic solvent is (0.1–0.2):1. The above lithium hydroxide solution is mixed with the phosphoric acid solution to obtain a crystal slurry, and then the above mixed solution is mixed with the crystal slurry to obtain a raw material solution. The raw material solution is subjected to an ultrasonic reaction at 0.2–1.6 MPa and 120–200°C, with an ultrasonic power of not less than 3 kW / m. 3After the ultrasonic reaction, the reaction system is subjected to solid-liquid separation to obtain crude lithium manganese iron phosphate and separation mother liquor. The crude lithium manganese iron phosphate is washed to obtain lithium manganese iron phosphate and a third mother liquor. Preferably, the sulfur content of lithium manganese iron phosphate is 0.1-0.5 wt%, and the conductivity of the third mother liquor is 100-1000 μS / cm. The separation mother liquor and the first mother liquor are subjected to first liquid phase recovery treatment to obtain a first recovered liquid phase. Preferably, the first recovered liquid phase can also be precision filtered to remove residual manganese and iron ions. Barium hydroxide octahydrate is mixed with deionized water to prepare a barium hydroxide octahydrate solution. Preferably, the barium hydroxide octahydrate solution can also be filtered to remove insoluble impurities in the solution. Preferably, the dissolution temperature of barium hydroxide octahydrate is 90-1000 μS / cm. At 100℃, the concentration of the barium hydroxide octahydrate solution is 20-50 wt%. The first recovered liquid phase is mixed with the barium hydroxide octahydrate solution for reaction, wherein the molar ratio of barium ions to sulfate ions is 1:(1-1.01), the reaction temperature is 15-90℃, and the reaction solution is subjected to a second liquid phase recovery treatment to obtain a second recovered liquid phase and barium sulfate precipitate. The second recovered liquid phase is concentrated and dried to obtain recovered lithium hydroxide monohydrate, which is used as a lithium source. Preferably, the concentration of lithium hydroxide in the concentrated second recovered liquid phase is 50-130 g / L. The barium sulfate precipitate is washed, dried, and pulverized to obtain barium sulfate product. Preferably, the median particle size D50 of the barium sulfate product is 0.6-0.8 μm, the purity is >99%, and the whiteness is >98%.

[0078] The present invention does not specifically limit the method of first liquid phase recovery treatment and second liquid phase recovery treatment. For example, the separation mother liquor and the first mother liquor can be subjected to first liquid phase recovery treatment and the reaction liquid can be subjected to second liquid phase recovery treatment through solid-liquid separation treatment. The solid-liquid separation treatment method can be at least one of diaphragm filter press, horizontal sedimentation screw centrifuge and disc centrifuge.

[0079] This invention does not specifically limit the method of precision filtration. For example, precision filtration can be performed using nanofiltration membranes and / or ion exchange resins.

[0080] The present invention does not specifically limit the concentration method. For example, concentration can be carried out by at least one of vacuum flash dryer, multi-effect evaporator, MVR evaporator and membrane concentration. Preferably, concentration is carried out by vacuum flash dryer, and the vacuum flash temperature is preferably 50-90°C.

[0081] The present invention does not specifically limit the drying method. For example, at least one of the following can be used for drying: vacuum flash dryer, double cone vacuum dryer, and fluidized bed dryer.

[0082] The present invention does not specifically limit the washing and pulverizing methods. For example, the washing and pulverizing methods defined above can be used, which will not be elaborated here.

[0083] To ensure the purity and electrochemical performance of lithium manganese iron phosphate, an excess of lithium source is often added during production. However, excessive lithium source can lead to incomplete reaction and waste. When the lithium source is lithium hydroxide monohydrate, the iron source is ferrous sulfate heptahydrate, the manganese source is manganese sulfate monohydrate, and the phosphorus source is phosphoric acid, a second recovered liquid phase containing lithium hydroxide can be obtained by reacting the first recovered liquid phase with a barium hydroxide octahydrate solution. After concentration and drying of the second recovered liquid phase, recovered lithium hydroxide can be obtained and recycled for use in the preparation of lithium source solutions, improving the lithium source recycling rate, avoiding raw material waste, and reducing costs. At the same time, barium sulfate, a high-performance byproduct, can also be obtained and used as a filler, coating, or catalyst.

[0084] Specifically, the lithium source cycle rate can be calculated using Equation 1:

[0085] Lithium source recycling rate (%) = M2 / (M0-M1)×100% Equation 1

[0086] In Formula 1, M0 is the actual amount of lithium source added, M1 is the theoretical amount of lithium source added, and M2 is the amount of lithium source recovered. The theoretical amount of lithium source added can be calculated based on the chemical formula of the lithium manganese iron phosphate to be prepared.

[0087] A second aspect of this invention provides a production system for carbon-coated lithium manganese iron phosphate (LMP) cathode active materials, applicable to the method for preparing carbon-coated LMP cathode active materials according to the first aspect. The system includes an ultrasonic mixed solvothermal reaction crystallization apparatus and a sintering apparatus. The ultrasonic mixed solvothermal reaction crystallization apparatus is used to prepare LMP, and the sintering apparatus is used to sinter LMP and a carbon source. The output port of the ultrasonic mixed solvothermal reaction crystallization apparatus is connected to the input port of the sintering apparatus. Since this production system is applicable to the method for preparing carbon-coated LMP cathode active materials according to the first aspect, it can produce small-particle-size LMP primary particles even with a high LMP solid content, thereby preparing high-performance carbon-coated LMP cathode active materials and achieving cost reduction and efficiency improvement.

[0088] In one specific embodiment, the carbon-coated lithium manganese iron phosphate cathode active material production system further includes a batching, dissolving, and purification device; the batching, dissolving, and purification device includes a dissolving module and a purification module; wherein, the output port of the dissolving module is connected to the input port of the purification module, and the output port of the purification module is connected to the input port of the ultrasonic mixing solvothermal reaction crystallization device.

[0089] It is understood that the dissolution module includes multiple dissolution containers for dissolving lithium, iron, manganese, and phosphorus sources respectively, and the purification module includes multiple filtration modules for removing insoluble impurities from the lithium, iron, manganese, and phosphorus source solutions. When the lithium source is lithium hydroxide monohydrate, the iron source is ferrous sulfate heptahydrate, the manganese source is manganese sulfate monohydrate, and the phosphorus source is phosphoric acid, the dissolution module also includes a dissolution container for dissolving barium hydroxide octahydrate, and the purification module also includes filtration modules for removing insoluble impurities from the barium hydroxide octahydrate solution.

[0090] Furthermore, the above-mentioned dissolution module also includes an ultrasonic loading module, which is used to load ultrasonic waves onto the ultrasonic dissolution container to make the solution disperse more evenly.

[0091] In one specific embodiment, the ultrasonic mixed solvothermal reaction crystallization device includes a mixing module and an ultrasonic mixed solvothermal reaction crystallization module; the input port of the mixing module is connected to the output port of the purification module, and the output port of the mixing module is connected to the input port of the ultrasonic mixed solvothermal reaction crystallization module. The mixing module is used to prepare the raw material solution, and the ultrasonic mixed solvothermal reaction crystallization module is used to perform an ultrasonic reaction on the raw material solution.

[0092] In one specific embodiment, the carbon-coated lithium manganese iron phosphate cathode active material production system further includes a slurry solid-liquid separation and washing device; the slurry solid-liquid separation and washing device includes a solid-liquid separation module and a washing module; the input port of the solid-liquid separation module is connected to the output port of the ultrasonic mixed solvothermal reaction crystallization module, the solid phase output port of the solid-liquid separation module is connected to the solid phase input port of the washing module, and the solid phase output port of the washing module is connected to the input port of the sintering device. The solid-liquid separation module is used to perform solid-liquid separation treatment on the reaction system after the ultrasonic reaction to obtain crude lithium manganese iron phosphate and separation mother liquor, and the washing module is used to wash the crude lithium manganese iron phosphate to obtain lithium manganese iron phosphate and washing mother liquor.

[0093] In one specific embodiment, the washing module includes a primary washing module, a first solid-liquid separation module, a secondary washing module, a second solid-liquid separation module, a tertiary washing module, and a third solid-liquid separation module. The solid phase input port of the primary washing module is connected to the solid phase output port of the solid-liquid separation module. The output port of the primary washing module is connected to the first solid phase input port of the secondary washing module through the first solid-liquid separation module. The output port of the secondary washing module is connected to the solid phase input port of the tertiary washing module through the second solid-liquid separation module. The output port of the tertiary washing module is connected to the input port of the third solid-liquid separation module. In one embodiment, the liquid phase output port of the third solid-liquid separation module is connected to the liquid phase input port of the secondary washing module, and the liquid phase output port of the second solid-liquid separation module is connected to the liquid phase input port of the primary washing module. It can be understood that the primary, secondary, and tertiary washing modules also include a washing liquid input port for inputting a washing liquid including deionized water.

[0094] Furthermore, the ultrasonic loading module in the primary, secondary, and tertiary washing modules ensures more thorough washing.

[0095] In one specific embodiment, the washing module further includes a filtration module for separating a small amount of lithium manganese iron phosphate from the separation mother liquor and the first mother liquor; the liquid phase output port of the solid-liquid separation module is connected to the first input port of the filtration module, the liquid phase output port of the first solid-liquid separation module is connected to the second input port of the filtration module, and the solid phase output port of the filtration module is connected to the second solid phase input port of the secondary washing module.

[0096] In one specific embodiment, the carbon-coated lithium manganese iron phosphate cathode active material production system further includes a mixing module, a grinding module, and a granulation module. The input port of the mixing module is connected to the solid phase output port of the third solid-liquid separation module, the output port of the mixing module is connected to the input port of the grinding module, and the output port of the grinding module is connected to the input port of the granulation module. The mixing module is used to uniformly mix lithium manganese iron phosphate, a carbon source, and deionized water to obtain a slurry; the grinding module is used to grind the slurry to obtain a crystal slurry; and the granulation module is used to granulate the crystal slurry to obtain lithium manganese iron phosphate mixed with carbon source particles.

[0097] Furthermore, the mixing module may also include an ultrasonic loading module to make the lithium manganese iron phosphate and carbon source more uniformly dispersed in deionized water.

[0098] In one specific embodiment, the sintering apparatus includes a sintering module and a first pulverizing module; the input port of the sintering module is connected to the output port of the granulation module, and the output port of the sintering module is connected to the input port of the first pulverizing module. The sintering module is used to sinter lithium manganese iron phosphate mixed carbon source particles to obtain a sintered product, and the first pulverizing module is used to pulverize the sintered product.

[0099] In one specific embodiment, the carbon-coated lithium manganese iron phosphate cathode active material production system further includes a precision filtration module, a barium sulfate reaction crystallization module, a barium sulfate crystal slurry solid-liquid separation module, a first drying module, and a second pulverizing module; the output port of the purification module is connected to the first input port of the barium sulfate reaction crystallization module, the input port of the precision filtration module is connected to the liquid phase output port of the filtration module, the output port of the precision filtration module is connected to the second input port of the barium sulfate reaction crystallization module, the output port of the barium sulfate reaction crystallization module is connected to the input port of the barium sulfate crystal slurry solid-liquid separation module, and the solid phase output port of the barium sulfate crystal slurry solid-liquid separation module is connected to the input port of the second pulverizing module via the first drying module. The system includes a precision filtration module for precisely filtering the first recovered liquid phase, a barium sulfate reaction crystallization module for reacting the barium hydroxide octahydrate solution with the first recovered liquid phase, a barium sulfate crystal slurry solid-liquid separation module for performing a second liquid phase recovery treatment on the reaction liquid of the barium hydroxide octahydrate solution and the first recovered liquid phase to obtain the second recovered liquid phase and barium sulfate precipitate, a first drying module for drying the barium sulfate precipitate to remove moisture, and a second pulverizing module for pulverizing the dried barium sulfate to obtain the barium sulfate product.

[0100] In one specific embodiment, the carbon-coated lithium manganese iron phosphate cathode active material production system further includes a lithium hydroxide recovery device; the lithium hydroxide recovery device includes a concentration module and a second drying module; the input port of the concentration module is connected to the liquid phase output port of the barium sulfate crystal slurry solid-liquid separation module, the output port of the concentration module is connected to the input port of the second drying module, and the output port of the drying module is connected to the input port of the dissolution module. The concentration module is used to concentrate the second recovered liquid phase, and the second drying module is used to dry the concentrated second recovered liquid phase.

[0101] The preparation method of the carbon-coated lithium manganese iron phosphate cathode active material of the present invention will be described in detail below through specific embodiments.

[0102] Example 1

[0103] (1) 1451.5 kg of lithium hydroxide monohydrate, 9600 L of deoxygenated deionized water and diethylene glycol (volume ratio 2.2:1) were added to the first ultrasonic dissolving container under nitrogen protection and stirred for 30 min at a dissolving temperature of 35°C. The dissolved solution was then passed through a ceramic precision filter to remove insoluble impurities, yielding a lithium hydroxide solution. 960.8 kg of ferrous sulfate heptahydrate, 1363.2 kg of manganese sulfate monohydrate, 50 g of ascorbic acid, and 3... 000L of deoxygenated deionized water was added to the second ultrasonic dissolving tank under nitrogen protection and stirred for 30 minutes at a dissolving temperature of 35℃. The dissolved solution was then passed through a ceramic precision filter to remove insoluble impurities, yielding a mixed solution of ferrous sulfate heptahydrate and manganese sulfate monohydrate. 788.4L of 85% thermal phosphoric acid and 6600L of diethylene glycol were added to the third ultrasonic dissolving tank and mixed evenly. The mixed solution was then passed through a ceramic precision filter to remove insoluble impurities, yielding a phosphoric acid solution.

[0104] (2) Lithium hydroxide solution and phosphoric acid solution are mixed in a first SK-type tubular static mixer to obtain lithium phosphate slurry. Then, the lithium phosphate slurry is further mixed with a mixed solution of ferrous sulfate heptahydrate and manganese sulfate monohydrate in a second SK-type tubular static mixer to obtain a raw material solution. The ratio of the molar amounts of lithium ions, iron ions, manganese ions, and phosphate ions in the raw material solution is maintained at 3:1:1. The raw material solution is then transported to an ultrasonic mixing solvothermal reaction crystallization device, sealed, and the atmosphere inside the device is replaced (using nitrogen to replace air). The reaction is carried out under ultrasonic action using inductive heating (ultrasonic power is 5kW / m). 3 The heating rate was set at 3℃ / min, the reaction temperature at 180℃, and the pressure at 1.0MPa. The reaction was stopped after 3 hours of constant temperature reaction. The solid content of lithium manganese iron phosphate in the reaction system was 80.6 kg / m³. 3 ;

[0105] (3) After the above reaction is completed, the temperature of the reaction liquid is reduced to below 60°C using a spiral tube heat exchanger. The cooled reaction liquid is then transported to a first horizontal spiral sedimentation centrifuge for solid-liquid separation, yielding a mother liquor and a crude lithium manganese iron phosphate product with a solid content of 45%. The crude lithium manganese iron phosphate product with a solid content of 45% is then transported to a primary washing tank and mixed with a first washing liquid containing deionized water. After the first washing is performed under ultrasonic and stirring conditions, the mixture is transported to a second horizontal spiral sedimentation centrifuge for the first solid-liquid separation, yielding a first mother liquor and a first crude lithium manganese iron phosphate product with a solid content of 45%. A 45% concentration of the crude lithium manganese iron phosphate product was mixed with a second washing solution including deionized water. After a second washing under ultrasonic and stirring conditions, the mixture was transferred to a third horizontal spiral sedimentation centrifuge for a second solid-liquid separation, yielding a second mother liquor and a crude lithium manganese iron phosphate product with a solid content of 45%. The crude lithium manganese iron phosphate product with a solid content of 45% was then mixed with a third washing solution including deionized water. After a third washing under ultrasonic and stirring conditions, the mixture was transferred to a fourth horizontal spiral sedimentation centrifuge for a third solid-liquid separation, yielding a third mother liquor and a lithium manganese iron phosphate (LiFe) product with a solid content of 45%. 0.3 Mn 0.7 PO4, wherein the median particle size of the primary particles included in the lithium manganese iron phosphate is 150 nm; the conductivity of the third mother liquor is tested to be 350 μS / cm, and the sulfur content of the lithium manganese iron phosphate is 0.25%; wherein, the third mother liquor is introduced into the second washing liquid and circulated to participate in the second washing, the second mother liquor is introduced into the first washing liquid and circulated to participate in the first washing, and after the first mother liquor and the separation mother liquor are mixed, a rotary ceramic membrane separator is used to retain the small amount of residual lithium manganese iron phosphate in the first mother liquor and the separation mother liquor, to obtain the first recovered liquid phase and a small amount of lithium manganese iron phosphate, wherein the small amount of lithium manganese iron phosphate obtained is transported to the secondary washing tank and circulated to participate in the second washing;

[0106] (4) The lithium manganese iron phosphate with a solid content of 45% obtained after washing was mixed with carbon source (glucose and graphene) and deoxygenated deionized water, and ultrasonically dispersed to obtain a slurry with a solid content of 0.35%, wherein the mass ratio of glucose to graphene was 99:1, and the mass ratio of lithium manganese iron phosphate to carbon source was 0.88:0.12; the slurry was ground and refined using a nano-grinding mill to obtain a crystal slurry with a median particle size D50 of 100nm and D97 of 200nm; the crystal slurry was granulated by a high-speed centrifugal spray dryer, with the inlet air temperature set at 250℃ and the outlet air temperature at 105℃, to obtain lithium manganese iron phosphate mixed carbon source particles with a median particle size of 13.5μm and a residual water content of <1%;

[0107] (5) The above-mentioned carbon source particles of lithium manganese iron phosphate were sintered in an atmosphere sintering rotary furnace. The sintering temperature was set at 700℃, the heating rate was 3℃ / min, and the constant temperature sintering time was 4h. The sintered product was obtained. The sintered product was crushed by ceramic flat airflow pulverizer to obtain carbon-coated lithium manganese iron phosphate positive electrode active material with median particle size D50 of 100nm and D97 of 350nm.

[0108] (6) Under nitrogen protection, barium hydroxide octahydrate and deionized water are mixed in the fourth ultrasonic dissolving tank and stirred to dissolve at a temperature of 90°C. The dissolved liquid is then filtered through a ceramic precision filter to remove insoluble impurities, resulting in a barium hydroxide solution with a mass percentage of 40 wt%. The first recovered liquid phase obtained in step (3) is further treated with ion exchange resin to remove residual iron and manganese ions. The first recovered liquid phase with removed iron and manganese ions is mixed with the barium hydroxide solution, maintaining a molar ratio of barium ions to sulfate ions of 1:1. The reaction is carried out at 30°C to obtain a reaction solution. The reaction solution is then treated with a diaphragm filter press for second liquid phase recovery to obtain barium sulfate precipitate and second recovered liquid phase. The barium sulfate precipitate is dried and crushed by a flash dryer and a flat air jet mill to obtain a barium sulfate product with a median particle size D50 of 0.68 μm, a purity of 99.3%, and a whiteness of 98.5%.

[0109] (7) The second recovered liquid phase was concentrated using a vacuum flash evaporator. The concentration of lithium hydroxide in the concentrated second recovered liquid phase was 90 g / L. The vacuum flash evaporation temperature was set to 60 °C. After drying, 959.0 kg of recovered lithium hydroxide monohydrate was obtained. The recovered lithium hydroxide monohydrate was used as the lithium source in step (1). The recycling rate of lithium hydroxide monohydrate was 99.1%.

[0110] Example 2

[0111] (1) 1815.0 kg of lithium hydroxide monohydrate, 9600 L of deoxygenated deionized water and diethylene glycol (volume ratio 2.2:1) were added to the first ultrasonic dissolving tank under nitrogen protection and stirred for 30 min at a dissolving temperature of 35°C. The dissolved solution was then passed through a ceramic precision filter to remove insoluble impurities, yielding a lithium hydroxide solution. 1201.2 kg of ferrous sulfate heptahydrate, 1704.5 kg of manganese sulfate monohydrate, and 3000 L of diethylene glycol were added to the first ultrasonic dissolving tank. Deoxygenated deionized water was added to the second ultrasonic dissolving tank under nitrogen protection and stirred for 30 minutes at a dissolving temperature of 35°C. The dissolved solution was then passed through a ceramic precision filter to remove insoluble impurities, yielding a mixed solution of ferrous sulfate heptahydrate and manganese sulfate monohydrate. 988.2 L of 85% thermal phosphoric acid and 6600 L of diethylene glycol were added to the third ultrasonic dissolving tank and mixed evenly. The mixed solution was then passed through a ceramic precision filter to remove insoluble impurities, yielding a phosphoric acid solution.

[0112] (2) Lithium hydroxide solution and phosphoric acid solution are mixed in a first SK-type tubular static mixer to obtain lithium phosphate slurry. Then, the lithium phosphate slurry is further mixed with a mixed solution of ferrous sulfate heptahydrate and manganese sulfate monohydrate in a second SK-type tubular static mixer to obtain a raw material solution. The ratio of the molar amounts of lithium ions, iron ions, manganese ions, and phosphate ions in the raw material solution is maintained at 3:1:1. The raw material solution is then transported to an ultrasonic mixing solvothermal reaction crystallization device, sealed, and the atmosphere inside the device is replaced (using nitrogen to replace air). The reaction is carried out under ultrasonic action using inductive heating (ultrasonic power is 3kW / m). 3 The heating rate was set at 5℃ / min, the reaction temperature at 160℃, and the pressure at 0.62MPa. The reaction was stopped after 3 hours of constant temperature reaction. The solid content of lithium manganese iron phosphate in the reaction system was 97.6 kg / m³. 3 ;

[0113] (3) After the above reaction is completed, the temperature of the reaction liquid is reduced to below 60°C using a spiral tube heat exchanger. The cooled reaction liquid is then transported to a first horizontal spiral sedimentation centrifuge for solid-liquid separation, yielding a mother liquor and a crude lithium manganese iron phosphate product with a solid content of 45%. The crude lithium manganese iron phosphate product with a solid content of 45% is then transported to a primary washing tank and mixed with a first washing liquid containing deionized water. After the first washing is performed under ultrasonic and stirring conditions, the mixture is transported to a second horizontal spiral sedimentation centrifuge for the first solid-liquid separation, yielding a first mother liquor and a first crude lithium manganese iron phosphate product with a solid content of 45%. A 45% concentration of the crude lithium manganese iron phosphate product was mixed with a second washing solution including deionized water. After a second washing under ultrasonic and stirring conditions, the mixture was transferred to a third horizontal spiral sedimentation centrifuge for a second solid-liquid separation, yielding a second mother liquor and a crude lithium manganese iron phosphate product with a solid content of 45%. The crude lithium manganese iron phosphate product with a solid content of 45% was then mixed with a third washing solution including deionized water. After a third washing under ultrasonic and stirring conditions, the mixture was transferred to a fourth horizontal spiral sedimentation centrifuge for a third solid-liquid separation, yielding a third mother liquor and a lithium manganese iron phosphate (LiFe) product with a solid content of 45%. 0.3 Mn 0.7 PO4, wherein the median particle size of the primary particles included in the lithium manganese iron phosphate is 120 nm; the conductivity of the third mother liquor is tested to be 300 μS / cm, and the sulfur content of the lithium manganese iron phosphate is 0.22%; wherein, the third mother liquor is introduced into the second washing liquid and circulated to participate in the second washing, the second mother liquor is introduced into the first washing liquid and circulated to participate in the first washing, and after the first mother liquor and the separation mother liquor are mixed, a rotary ceramic membrane separator is used to retain the small amount of residual lithium manganese iron phosphate in the first mother liquor and the separation mother liquor, to obtain the first recovered liquid phase and a small amount of lithium manganese iron phosphate, wherein the small amount of lithium manganese iron phosphate obtained is transported to the secondary washing tank and circulated to participate in the second washing;

[0114] (4) The lithium manganese iron phosphate with a solid content of 45% obtained after washing was mixed with the carbon source glucose, the catalyst nano silver, and deoxygenated deionized water, and ultrasonically dispersed to obtain a slurry with a solid content of 0.35%, wherein the mass ratio of glucose to nano silver was 98:2, and the mass ratio of lithium manganese iron phosphate to carbon source was 0.9:0.1; the slurry was ground and refined using a nano milling machine to obtain a crystal slurry with a median particle size D50 of 120nm and D97 = 250nm; the crystal slurry was granulated by a high-speed centrifugal spray dryer, and the inlet air temperature was set to 260℃ and the outlet air temperature was set to 110℃ to obtain lithium manganese iron phosphate mixed carbon source particles with a median particle size of 12.5μm and a residual water content of <1%;

[0115] (5) The above-mentioned lithium manganese iron phosphate mixed carbon source particles were sintered in an atmosphere sintering rotary furnace. The sintering temperature was set at 650℃, the heating rate was 3℃ / min, and the constant temperature sintering time was 5h. The sintered product was obtained. The sintered product was crushed by ceramic flat airflow pulverizer to obtain carbon-coated lithium manganese iron phosphate positive electrode active material with median particle size D50 of 120nm and D97 of 400nm.

[0116] (6) Under nitrogen protection, barium hydroxide octahydrate and deionized water are mixed in the fourth ultrasonic dissolving tank and stirred to dissolve at a temperature of 90°C. The dissolved liquid is then filtered through a ceramic precision filter to remove insoluble impurities, resulting in a barium hydroxide solution with a mass percentage of 35 wt%. The first recovered liquid phase obtained in step (3) is further treated with ion exchange resin to remove residual iron and manganese ions. The first recovered liquid phase with removed iron and manganese ions is mixed with the barium hydroxide solution, maintaining a molar ratio of barium ions to sulfate ions of 1:1. The reaction is carried out at 30°C to obtain a reaction solution. The reaction solution is then treated with a diaphragm filter press to recover the second liquid phase, resulting in barium sulfate precipitate and the second recovered liquid phase. The barium sulfate precipitate is dried and crushed by a flash dryer and a flat air jet mill, respectively, to obtain a barium sulfate product with a median particle size D50 of 0.72 μm, a purity of 99.2%, and a whiteness of 98.7%.

[0117] (7) The second recovered liquid phase was concentrated using a vacuum flash evaporator. The concentration of lithium hydroxide in the concentrated second recovered liquid phase was 90 g / L. The vacuum flash evaporation temperature was set to 70 °C. After drying, 1197.9 kg of recovered lithium hydroxide monohydrate was obtained. The recovered lithium hydroxide monohydrate was used as the lithium source in step (1). The recycling rate of lithium hydroxide monohydrate was 99.0%.

[0118] Example 3

[0119] (1) 1440.0 kg of lithium hydroxide monohydrate, 9600 L of deoxygenated deionized water and diethylene glycol (volume ratio 2.2:1) were added to the first ultrasonic dissolving container under nitrogen protection and stirred for 30 min at a dissolving temperature of 35 °C. The dissolved solution was then passed through a ceramic precision filter to remove insoluble impurities, yielding a lithium hydroxide solution. 1059.4 kg of ferrous sulfate heptahydrate, 1503.3 kg of manganese sulfate monohydrate, and 50 g of ascorbic acid were added to the solution. 3000L of deoxygenated deionized water was added to the second ultrasonic dissolving tank under nitrogen protection and stirred for 30 minutes at a dissolving temperature of 35℃. The dissolved solution was then passed through a ceramic precision filter to remove insoluble impurities, yielding a mixed solution of ferrous sulfate heptahydrate and manganese sulfate monohydrate. 871.6L of 85% thermal phosphoric acid and 6600L of diethylene glycol were added to the third ultrasonic dissolving tank and mixed evenly. The mixed solution was then passed through a ceramic precision filter to remove insoluble impurities, yielding a phosphoric acid solution.

[0120] (2) Lithium hydroxide solution and phosphoric acid solution are mixed in a first SK-type tubular static mixer to obtain lithium phosphate slurry. Then, the lithium phosphate slurry is further mixed with a mixed solution of ferrous sulfate heptahydrate and manganese sulfate monohydrate in a second SK-type tubular static mixer to obtain a raw material solution. The ratio of the molar amounts of lithium ions, the total molar amounts of iron and manganese ions, and the molar amounts of phosphate ions in the raw material solution is maintained at 2.7:1:1. The raw material solution is then transported to an ultrasonic mixing solvothermal reaction crystallization device, sealed, and the atmosphere inside the device is replaced (using nitrogen to replace air). The reaction is carried out under ultrasonic action using inductive heating (ultrasonic power is 4kW / m). 3 The heating rate was set at 0.5℃ / min, the reaction temperature at 200℃, and the pressure at 1.6MPa. The reaction was stopped after 0.5 hours at a constant temperature. The solid content of lithium manganese iron phosphate in the reaction system was 88.3 kg / m³. 3 ;

[0121] (3) After the above reaction is completed, the temperature of the reaction liquid is reduced to below 60°C using a spiral tube heat exchanger. The cooled reaction liquid is then transported to a first horizontal spiral sedimentation centrifuge for solid-liquid separation, yielding a mother liquor and a crude lithium manganese iron phosphate product with a solid content of 45%. The crude lithium manganese iron phosphate product with a solid content of 45% is then transported to a primary washing tank and mixed with a first washing liquid containing deionized water. After the first washing is performed under ultrasonic and stirring conditions, the mixture is transported to a second horizontal spiral sedimentation centrifuge for the first solid-liquid separation, yielding a first mother liquor and a first crude lithium manganese iron phosphate product with a solid content of 45%. A 45% concentration of the crude lithium manganese iron phosphate product was mixed with a second washing solution including deionized water. After a second washing under ultrasonic and stirring conditions, the mixture was transferred to a third horizontal spiral sedimentation centrifuge for a second solid-liquid separation, yielding a second mother liquor and a crude lithium manganese iron phosphate product with a solid content of 45%. The crude lithium manganese iron phosphate product with a solid content of 45% was then mixed with a third washing solution including deionized water. After a third washing under ultrasonic and stirring conditions, the mixture was transferred to a fourth horizontal spiral sedimentation centrifuge for a third solid-liquid separation, yielding a third mother liquor and a lithium manganese iron phosphate (LiFe) product with a solid content of 45%. 0.3 Mn 0.7 PO4, wherein the median particle size of the primary particles included in the lithium manganese iron phosphate is 200 nm; the conductivity of the third mother liquor is tested to be 350 μS / cm, and the sulfur content of the lithium manganese iron phosphate is 0.25%; wherein, the third mother liquor is introduced into the second washing liquid and circulated to participate in the second washing, the second mother liquor is introduced into the first washing liquid and circulated to participate in the first washing, and after the first mother liquor and the separation mother liquor are mixed, a rotary ceramic membrane separator is used to retain the small amount of residual lithium manganese iron phosphate in the first mother liquor and the separation mother liquor, to obtain the first recovered liquid phase and a small amount of lithium manganese iron phosphate, wherein the small amount of lithium manganese iron phosphate obtained is transported to the secondary washing tank and circulated to participate in the second washing;

[0122] (4) The lithium manganese iron phosphate with a solid content of 45% obtained after washing was mixed with carbon source (glucose and graphene) and deoxygenated deionized water, and ultrasonically dispersed to obtain a slurry with a solid content of 0.35, wherein the mass ratio of glucose to graphene was 99:1, and the mass ratio of lithium manganese iron phosphate to carbon source was 0.85:0.15; the slurry was ground and refined using a nano-grinding mill to obtain a crystal slurry with a median particle size D50 of 180nm and D97 = 345nm; the crystal slurry was granulated by a two-fluid spray dryer, and the inlet air temperature was set to 250℃ and the outlet air temperature was set to 105℃ to obtain lithium manganese iron phosphate mixed carbon source particles with a median particle size of 4.5μm and a residual water content of <1%;

[0123] (5) The above-mentioned lithium manganese iron phosphate mixed carbon source particles were sintered in an atmosphere sintering rotary furnace. The sintering temperature was set at 750℃, the heating rate was 1.0℃ / min, and the constant temperature sintering time was 1h. The sintered product was obtained. The sintered product was crushed by ceramic flat airflow pulverizer to obtain carbon-coated lithium manganese iron phosphate positive electrode active material with median particle size D50 of 185m and D97 of 395nm.

[0124] (6) Under nitrogen protection, barium hydroxide octahydrate and deionized water are mixed in the fourth ultrasonic dissolving tank and stirred to dissolve at a temperature of 90°C. The dissolved liquid is then filtered through a ceramic precision filter to remove insoluble impurities, resulting in a barium hydroxide solution with a mass percentage of 40 wt%. The first recovered liquid phase obtained in step (3) is further treated with ion exchange resin to remove residual iron and manganese ions. The first recovered liquid phase with removed iron and manganese ions is mixed with the barium hydroxide solution, maintaining a molar ratio of barium ions to sulfate ions of 1:1. The reaction is carried out at 30°C to obtain a reaction solution. The reaction solution is then treated with a diaphragm filter press for second liquid phase recovery to obtain barium sulfate precipitate and second recovered liquid phase. The barium sulfate precipitate is dried and crushed by a flash dryer and a flat air jet mill, respectively, to obtain a barium sulfate product with a median particle size D50 of 0.72 μm, a purity of 99.5%, and a whiteness of 98.7%.

[0125] (7) The second recovered liquid phase was concentrated using a vacuum flash evaporator. The concentration of lithium hydroxide in the concentrated second recovered liquid phase was 90 g / L. The vacuum flash evaporation temperature was set to 65°C. After drying, 897.6 kg of recovered lithium hydroxide monohydrate was obtained. The recovered lithium hydroxide monohydrate was used as the lithium source in step (1). The recycling rate of lithium hydroxide monohydrate was 99.0%.

[0126] Example 4

[0127] (1) 1936.0 kg of lithium hydroxide monohydrate, 9600 L of deoxygenated deionized water and diethylene glycol (volume ratio 2.2:1) were added to the first ultrasonic dissolving container under nitrogen protection and stirred for 30 min at a dissolving temperature of 35 °C. The dissolved solution was then passed through a ceramic precision filter to remove insoluble impurities, yielding a lithium hydroxide solution. 1201.2 kg of ferrous sulfate heptahydrate, 1704.1 kg of manganese sulfate monohydrate, and 50 g of ascorbic acid were added to the solution. 3000L of deoxygenated deionized water was added to the second ultrasonic dissolving tank under nitrogen protection and stirred for 30 minutes at a dissolving temperature of 35℃. The dissolved solution was then passed through a ceramic precision filter to remove insoluble impurities, yielding a mixed solution of ferrous sulfate heptahydrate and manganese sulfate monohydrate. 988.2L of 85% thermal phosphoric acid and 6600L of diethylene glycol were added to the third ultrasonic dissolving tank and mixed evenly. The mixed solution was then passed through a ceramic precision filter to remove insoluble impurities, yielding a phosphoric acid solution.

[0128] (2) Lithium hydroxide solution and phosphoric acid solution were mixed in a first SK-type tubular static mixer to obtain lithium phosphate slurry. Then, the lithium phosphate slurry was further mixed with a mixed solution of ferrous sulfate heptahydrate and manganese sulfate monohydrate in a second SK-type tubular static mixer to obtain a raw material solution. The ratio of the molar amounts of lithium ions, iron ions, and manganese ions, and the molar amounts of phosphate ions in the raw material solution was maintained at 3.2:1:1. The raw material solution was then transported to an ultrasonic mixing solvothermal reaction crystallization device, sealed, and the atmosphere inside the device was replaced with nitrogen. The air was replaced with nitrogen, and the reaction was carried out under ultrasonic action using inductive heating. The heating rate was set at 20℃ / min, the reaction temperature at 160℃, and the pressure at 0.62MPa. The reaction was stopped after 24 hours of constant temperature reaction. The solid content of lithium manganese iron phosphate in the reaction system was 97.3 kg / m³. 3 ;

[0129] (3) After the above reaction is completed, the temperature of the reaction liquid is reduced to below 60°C using a spiral tube heat exchanger. The cooled reaction liquid is then transported to a first horizontal spiral sedimentation centrifuge for solid-liquid separation to obtain a mother liquor and a crude lithium manganese iron phosphate product with a solid content of 45%. The crude lithium manganese iron phosphate product with a solid content of 45% is then transported to a primary washing tank and mixed with a first washing liquid including deionized water. After a first washing under ultrasonic and stirring action, it is transported to a second horizontal spiral sedimentation centrifuge for the first solid-liquid separation to obtain a first mother liquor and a first crude lithium manganese iron phosphate product with a solid content of 45%. The first crude lithium manganese iron phosphate product with a solid content of 45% is then mixed with a second washing liquid including deionized water. After a second washing under ultrasonic and stirring action, it is transported to a third horizontal spiral sedimentation centrifuge for the second solid-liquid separation to obtain a second mother liquor and a second crude lithium manganese iron phosphate product with a solid content of 45%. The crude lithium manganese iron phosphate (LMFP) product, containing 45% of the first grade, is mixed with a third washing solution including deionized water. After a third washing under ultrasonic and stirring conditions, the mixture is transferred to a fourth horizontal spiral sedimentation centrifuge for a third solid-liquid separation, yielding a third mother liquor and LFP with a solid content of 45%. The median particle size of the primary particles in the LFP is 160 nm. The conductivity of the third mother liquor is 350 μS / cm, and the sulfur content of the LFP is 0.25%. The third mother liquor is then introduced into the second washing solution for recycling during the second washing process. The second mother liquor is also introduced into the first washing solution for recycling during the first washing process. After mixing the first mother liquor and the separated mother liquor, a rotary ceramic membrane separator is used to remove any remaining trace amounts of LFP from both the first mother liquor and the separated mother liquor, resulting in a first recovered liquid phase and a small amount of LFP. The remaining trace amount of LFP is then transferred to a secondary washing tank for recycling during the second washing process.

[0130] (4) The lithium manganese iron phosphate with a solid content of 45% obtained after washing was mixed with carbon source (glucose and graphene) and deoxygenated deionized water, and ultrasonically dispersed to obtain a slurry with a solid content of 0.35, wherein the mass ratio of glucose to graphene was 99:1, and the mass ratio of lithium manganese iron phosphate to carbon source was 0.95:0.05; the slurry was ground and refined using a nano-grinding mill to obtain a crystal slurry with a median particle size D50 of 150nm and D97 = 345nm; the crystal slurry was granulated by a three-fluid spray dryer, and the inlet air temperature was set to 250℃ and the outlet air temperature was set to 105℃ to obtain lithium manganese iron phosphate mixed carbon source particles with a median particle size of 5.5μm and a residual water content of <1%;

[0131] (5) The above-mentioned lithium manganese iron phosphate mixed carbon source particles were sintered in an atmosphere sintering rotary furnace. The sintering temperature was set at 650℃, the heating rate was 3℃ / min, and the constant temperature sintering time was 6h. The sintered product was obtained. The sintered product was crushed by ceramic flat airflow pulverizer to obtain carbon-coated lithium manganese iron phosphate positive electrode active material with median particle size D50 of 165nm and D97 of 390nm.

[0132] (6) Under nitrogen protection, barium hydroxide octahydrate and deionized water are mixed in the fourth ultrasonic dissolving tank and stirred to dissolve at a temperature of 90°C. The dissolved liquid is then filtered through a ceramic precision filter to remove insoluble impurities, resulting in a barium hydroxide solution with a mass percentage of 40 wt%. The first recovered liquid phase obtained in step (3) is further treated with ion exchange resin to remove residual iron and manganese ions. The first recovered liquid phase with removed iron and manganese ions is mixed with the barium hydroxide solution, maintaining a molar ratio of barium ions to sulfate ions of 1:1. The reaction is carried out at 30°C to obtain a reaction solution. The reaction solution is then treated with a diaphragm filter press for second liquid phase recovery to obtain barium sulfate precipitate and second recovered liquid phase. The barium sulfate precipitate is dried and crushed by a flash dryer and a flat air jet mill to obtain a barium sulfate product with a median particle size D50 of 0.75 μm, a purity of 99.2%, and a whiteness of 98.1%.

[0133] (7) The second recovered liquid phase was concentrated using a vacuum flash evaporator. The concentration of lithium hydroxide in the concentrated second recovered liquid phase was 90 g / L. The vacuum flash evaporation temperature was set to 60 °C. After drying, 1324.3 kg of recovered lithium hydroxide monohydrate was obtained. The recovered lithium hydroxide monohydrate was used as the lithium source in step (1). The recycling rate of lithium hydroxide monohydrate was 99.5%.

[0134] Example 5

[0135] (1) 1206.5 kg of lithium hydroxide monohydrate, 9600 L of deoxygenated deionized water and diethylene glycol (volume ratio 2.2:1) were added to the first ultrasonic dissolving container under nitrogen protection and stirred for 30 min at a dissolving temperature of 35°C. The dissolved solution was then passed through a ceramic precision filter to remove insoluble impurities, yielding a lithium hydroxide solution. 960.8 kg of ferrous sulfate heptahydrate, 1363.2 kg of manganese sulfate monohydrate, 50 g of ascorbic acid, and 3... 000L of deoxygenated deionized water was added to the second ultrasonic dissolving tank under nitrogen protection and stirred for 30 minutes at a dissolving temperature of 35℃. The dissolved solution was then passed through a ceramic precision filter to remove insoluble impurities, yielding a mixed solution of ferrous sulfate heptahydrate and manganese sulfate monohydrate. 788.4L of 85% thermal phosphoric acid and 6600L of diethylene glycol were added to the third ultrasonic dissolving tank and mixed evenly. The mixed solution was then passed through a ceramic precision filter to remove insoluble impurities, yielding a phosphoric acid solution.

[0136] (2) Lithium hydroxide solution and phosphoric acid solution are mixed in a first SK type tubular static mixer to obtain lithium phosphate slurry. Then, the lithium phosphate slurry is further mixed and reacted with a mixed solution of ferrous sulfate heptahydrate and manganese sulfate monohydrate in a second SK type tubular static mixer to obtain raw material solution. The ratio of the molar amounts of lithium ions, the total molar amounts of iron ions and manganese ions, and the molar amounts of phosphate ions in the raw material solution is maintained at 2.5:1:1. The raw material solution is then transported to an ultrasonic mixing solvothermal reaction crystallization device, sealed, and the atmosphere inside the device is replaced with nitrogen to replace the air. The reaction is carried out under ultrasonic action using inductive heating (ultrasonic power is 5KW / m). 3 The heating rate was set at 3℃ / min, the reaction temperature at 180℃, and the pressure at 1.0MPa. The reaction was stopped after 3 hours of constant temperature reaction. The solid content of lithium manganese iron phosphate in the reaction system was 81.3 kg / m³. 3 ;

[0137] (3) After the above reaction is completed, the temperature of the reaction liquid is reduced to below 60°C using a spiral tube heat exchanger. The cooled reaction liquid is then transported to a first horizontal spiral sedimentation centrifuge for solid-liquid separation, yielding a mother liquor and a crude lithium manganese iron phosphate product with a solid content of 45%. The crude lithium manganese iron phosphate product with a solid content of 45% is then transported to a primary washing tank and mixed with a first washing liquid containing deionized water. After the first washing is performed under ultrasonic and stirring conditions, the mixture is transported to a second horizontal spiral sedimentation centrifuge for the first solid-liquid separation, yielding a first mother liquor and a first crude lithium manganese iron phosphate product with a solid content of 45%. A 45% concentration of the crude lithium manganese iron phosphate product was mixed with a second washing solution including deionized water. After a second washing under ultrasonic and stirring conditions, the mixture was transferred to a third horizontal spiral sedimentation centrifuge for a second solid-liquid separation, yielding a second mother liquor and a crude lithium manganese iron phosphate product with a solid content of 45%. The crude lithium manganese iron phosphate product with a solid content of 45% was then mixed with a third washing solution including deionized water. After a third washing under ultrasonic and stirring conditions, the mixture was transferred to a fourth horizontal spiral sedimentation centrifuge for a third solid-liquid separation, yielding a third mother liquor and a lithium manganese iron phosphate (LiFe) product with a solid content of 45%. 0.3 Mn 0.7 PO4, wherein the median particle size of the primary particles included in the lithium manganese iron phosphate is 120 nm; the conductivity of the third mother liquor is tested to be 350 μS / cm, and the sulfur content of the lithium manganese iron phosphate is 0.25%; wherein the third mother liquor is introduced into the second washing liquid and circulated to participate in the second washing, the second mother liquor is introduced into the first washing liquid and circulated to participate in the first washing, and after the first mother liquor and the separation mother liquor are mixed, a rotary ceramic membrane separator is used to retain the small amount of residual lithium manganese iron phosphate in the first mother liquor and the separation mother liquor, to obtain the first recovered liquid phase and a small amount of lithium manganese iron phosphate, wherein the small amount of lithium manganese iron phosphate obtained is transported to the secondary washing tank and circulated to participate in the second washing;

[0138] (4) The lithium manganese iron phosphate with a solid content of 45% obtained after washing was mixed with carbon source (glucose and graphene) and deoxygenated deionized water, and ultrasonically dispersed to obtain a slurry with a solid content of 0.35%, wherein the mass ratio of glucose to graphene was 99:1, and the mass ratio of lithium manganese iron phosphate to carbon source was 0.88:0.12; the slurry was ground and refined using a nano-grinding mill to obtain a crystal slurry with a median particle size D50 of 150nm and D97 of 380nm; the crystal slurry was granulated by a high-speed centrifugal spray dryer, with the inlet air temperature set at 250℃ and the outlet air temperature at 105℃, to obtain secondary lithium manganese iron phosphate particles with a median particle size of 16.5μm and a residual water content of <1%;

[0139] (5) The above-mentioned secondary particles of lithium manganese iron phosphate were sintered in an atmosphere sintering rotary furnace. The sintering temperature was set at 700℃, the heating rate was 3℃ / min, and the constant temperature sintering time was 4h. The sintered product was obtained. The sintered product was crushed by ceramic flat airflow pulverizer to obtain carbon-coated lithium manganese iron phosphate positive electrode active material with median particle size D50 of 160nm and D97 of 400nm.

[0140] (6) Under nitrogen protection, barium hydroxide octahydrate and deionized water are mixed in the fourth ultrasonic dissolving tank and stirred to dissolve at a temperature of 90°C. The dissolved liquid is then filtered through a ceramic precision filter to remove insoluble impurities, resulting in a barium hydroxide solution with a mass percentage of 40 wt%. The first recovered liquid phase obtained in step (3) is further treated with ion exchange resin to remove residual iron and manganese ions. The first recovered liquid phase with removed iron and manganese ions is mixed with the barium hydroxide solution, maintaining a molar ratio of barium ions to sulfate ions of 1:1. The reaction is carried out at 30°C to obtain a reaction solution. The reaction solution is then treated with a diaphragm filter press for second liquid phase recovery to obtain barium sulfate precipitate and second recovered liquid phase. The barium sulfate precipitate is dried and crushed by a flash dryer and a flat air jet mill to obtain a barium sulfate product with a median particle size D50 of 0.69 μm, a purity of 99.0%, and a whiteness of 98.0%.

[0141] (7) The second recovered liquid phase was concentrated using a vacuum flash evaporator. The concentration of lithium hydroxide in the concentrated second recovered liquid phase was 90 g / L. The vacuum flash evaporation temperature was set to 66°C. After drying, 718.8 kg of recovered lithium hydroxide monohydrate was obtained. The recovered lithium hydroxide monohydrate was used as the lithium source in step (1). The recycling rate of lithium hydroxide monohydrate was 99.3%.

[0142] Example 6

[0143] (1) 1451.5 kg of lithium hydroxide monohydrate, 9600 L of deoxygenated deionized water and diethylene glycol (volume ratio 2.2:1) were added to the first ultrasonic dissolving container under nitrogen protection and stirred for 30 min at a dissolving temperature of 35°C. The dissolved solution was then passed through a ceramic precision filter to remove insoluble impurities, yielding a lithium hydroxide solution. 960.8 kg of ferrous sulfate heptahydrate, 1363.2 kg of manganese sulfate monohydrate, 50 g of ascorbic acid, and 3... 000L of deoxygenated deionized water was added to the second ultrasonic dissolving tank under nitrogen protection and stirred for 30 minutes at a dissolving temperature of 35℃. The dissolved solution was then passed through a ceramic precision filter to remove insoluble impurities, yielding a mixed solution of ferrous sulfate heptahydrate and manganese sulfate monohydrate. 788.4L of 85% thermal phosphoric acid and 6600L of diethylene glycol were added to the third ultrasonic dissolving tank and mixed evenly. The mixed solution was then passed through a ceramic precision filter to remove insoluble impurities, yielding a phosphoric acid solution.

[0144] (2) Lithium hydroxide solution and phosphoric acid solution are mixed in a first SK-type tubular static mixer to obtain lithium phosphate slurry. Then, the lithium phosphate slurry is further mixed with a mixed solution of ferrous sulfate heptahydrate and manganese sulfate monohydrate in a second SK-type tubular static mixer to obtain a raw material solution. The ratio of the molar amounts of lithium ions, iron ions, manganese ions, and phosphate ions in the raw material solution is maintained at 3:1:1. The raw material solution is then transported to an ultrasonic mixing solvothermal reaction crystallization device, sealed, and the atmosphere inside the device is replaced with nitrogen to replace the air. The reaction is carried out under ultrasonic action using inductive heating (ultrasonic power is 5KW / m). 3 The heating rate was set at 22℃ / min, the reaction temperature at 180℃, and the pressure at 1.0 MPa. The reaction was stopped after 26 hours of constant temperature reaction. The solid content of lithium manganese iron phosphate in the reaction system was 80.6 kg / m³. 3 ;

[0145] (3) After the above reaction is completed, the temperature of the reaction liquid is reduced to below 60°C using a spiral tube heat exchanger. The cooled reaction liquid is then transported to a first horizontal spiral sedimentation centrifuge for solid-liquid separation to obtain a mother liquor and a crude lithium manganese iron phosphate product with a solid content of 45%. The crude lithium manganese iron phosphate product with a solid content of 45% is then transported to a primary washing tank and mixed with a first washing liquid including deionized water. After a first washing under ultrasonic and stirring action, it is transported to a second horizontal spiral sedimentation centrifuge for the first solid-liquid separation to obtain a first mother liquor and a first crude lithium manganese iron phosphate product with a solid content of 45%. The first crude lithium manganese iron phosphate product with a solid content of 45% is then mixed with a second washing liquid including deionized water. After a second washing under ultrasonic and stirring action, it is transported to a third horizontal spiral sedimentation centrifuge for the second solid-liquid separation to obtain a second mother liquor and a second crude lithium manganese iron phosphate product with a solid content of 45%. The crude lithium manganese iron phosphate (LMFP) product, containing 45% of the first grade, is mixed with a third washing solution including deionized water. After a third washing under ultrasonic and stirring conditions, the mixture is transferred to a fourth horizontal spiral sedimentation centrifuge for a third solid-liquid separation, yielding a third mother liquor and LFP with a solid content of 45%. The median particle size of the primary particles in the LFP is 80 nm. The conductivity of the third mother liquor is 350 μS / cm, and the sulfur content of the LFP is 0.25%. The third mother liquor is then introduced into the second washing solution for recycling during the second washing process. The second mother liquor is also introduced into the first washing solution for recycling during the first washing process. After mixing the first mother liquor and the separated mother liquor, a rotary ceramic membrane separator is used to remove any residual LFP from both the first mother liquor and the separated mother liquor, yielding a first recovered liquid phase and a small amount of LFP. The small amount of LFP obtained is then transferred to a secondary washing tank for recycling during the second washing process.

[0146] (4) The lithium manganese iron phosphate with a solid content of 45% obtained after washing was mixed with carbon source (glucose and graphene) and deoxygenated deionized water, and ultrasonically dispersed to obtain a slurry with a solid content of 0.35%, wherein the mass ratio of glucose to graphene was 99:1, and the mass ratio of lithium manganese iron phosphate to carbon source was 0.88:0.12; the slurry was ground and refined using a nano-grinding mill to obtain a crystal slurry with a median particle size D50 of 80nm and D97 = 320nm; the crystal slurry was granulated by a high-speed centrifugal spray dryer, with the inlet air temperature set at 250℃ and the outlet air temperature at 105℃, to obtain secondary lithium manganese iron phosphate particles with a median particle size of 12μm and a residual water content of <1%;

[0147] (5) The above-mentioned secondary particles of lithium manganese iron phosphate were sintered in an atmosphere sintering rotary furnace. The sintering temperature was set at 700℃, the heating rate was 3℃ / min, and the constant temperature sintering time was 4h. The sintered product was obtained. The sintered product was crushed by ceramic flat airflow pulverizer to obtain carbon-coated lithium manganese iron phosphate positive electrode active material with median particle size D50 of 90nm and D97 of 350nm.

[0148] (6) Under nitrogen protection, barium hydroxide octahydrate and deionized water are mixed in the fourth ultrasonic dissolving tank and stirred to dissolve at a temperature of 90°C. The dissolved liquid is then filtered through a ceramic precision filter to remove insoluble impurities, resulting in a barium hydroxide solution with a mass percentage of 40 wt%. The first recovered liquid phase obtained in step (3) is further treated with ion exchange resin to remove residual iron and manganese ions. The first recovered liquid phase with removed iron and manganese ions is mixed with the barium hydroxide solution, maintaining a molar ratio of barium ions to sulfate ions of 1:1. The reaction is carried out at 30°C to obtain a reaction solution. The reaction solution is then treated with a diaphragm filter press for second liquid phase recovery to obtain barium sulfate precipitate and second recovered liquid phase. The barium sulfate precipitate is dried and crushed by a flash dryer and a flat air jet mill to obtain a barium sulfate product with a median particle size D50 of 0.7 μm, a purity of 99.5%, and a whiteness of 98.6%.

[0149] (7) The second recovered liquid phase was concentrated using a vacuum flash evaporator. The concentration of lithium hydroxide in the concentrated second recovered liquid phase was 90 g / L. The vacuum flash evaporation temperature was set to 60 °C. After drying, 959.9 kg of recovered lithium hydroxide monohydrate was obtained. The recovered lithium hydroxide monohydrate was used as the lithium source in step (1). The recycling rate of lithium hydroxide monohydrate was 99.2%.

[0150] Example 7

[0151] (1) 1451.5 kg of lithium hydroxide monohydrate, 9600 L of deoxygenated deionized water and diethylene glycol (volume ratio 2.2:1) were added to the first ultrasonic dissolving container under nitrogen protection and stirred for 30 min at a dissolving temperature of 35°C. The dissolved solution was then passed through a ceramic precision filter to remove insoluble impurities, yielding a lithium hydroxide solution. 960.8 kg of ferrous sulfate heptahydrate, 1363.2 kg of manganese sulfate monohydrate, 50 g of ascorbic acid, and 3... 000L of deoxygenated deionized water was added to the second ultrasonic dissolving tank under nitrogen protection and stirred for 30 minutes at a dissolving temperature of 35℃. The dissolved solution was then passed through a ceramic precision filter to remove insoluble impurities, yielding a mixed solution of ferrous sulfate heptahydrate and manganese sulfate monohydrate. 788.4L of 85% thermal phosphoric acid and 6600L of diethylene glycol were added to the third ultrasonic dissolving tank and mixed evenly. The mixed solution was then passed through a ceramic precision filter to remove insoluble impurities, yielding a phosphoric acid solution.

[0152] (2) Lithium hydroxide solution and phosphoric acid solution are mixed in a first SK-type tubular static mixer to obtain lithium phosphate slurry. Then, the lithium phosphate slurry is further mixed with a mixed solution of ferrous sulfate heptahydrate and manganese sulfate monohydrate in a second SK-type tubular static mixer to obtain a raw material solution. The ratio of the molar amounts of lithium ions, iron ions, manganese ions, and phosphate ions in the raw material solution is maintained at 3:1:1. The raw material solution is then transported to an ultrasonic mixing solvothermal reaction crystallization device, sealed, and the atmosphere inside the device is replaced with nitrogen to replace the air. The reaction is carried out under ultrasonic action using inductive heating (ultrasonic power is 5KW / m). 3 The heating rate was set at 3℃ / min, the reaction temperature at 180℃, and the pressure at 1.0MPa. The reaction was stopped after 3 hours of constant temperature reaction. The solid content of lithium manganese iron phosphate in the reaction system was 80.6 kg / m³. 3 ;

[0153] (3) After the above reaction is completed, the temperature of the reaction liquid is reduced to below 60°C using a spiral tube heat exchanger. The cooled reaction liquid is then transported to a first horizontal spiral sedimentation centrifuge for solid-liquid separation to obtain a mother liquor and a crude lithium manganese iron phosphate product with a solid content of 45%. The crude lithium manganese iron phosphate product with a solid content of 45% is then transported to a primary washing tank and mixed with a first washing liquid including deionized water. After a first washing under ultrasonic and stirring action, it is transported to a second horizontal spiral sedimentation centrifuge for the first solid-liquid separation to obtain a first mother liquor and a first crude lithium manganese iron phosphate product with a solid content of 45%. The first crude lithium manganese iron phosphate product with a solid content of 45% is then mixed with a second washing liquid including deionized water. After a second washing under ultrasonic and stirring action, it is transported to a third horizontal spiral sedimentation centrifuge for the second solid-liquid separation to obtain a second mother liquor and a second crude lithium manganese iron phosphate product with a solid content of 45%. The crude lithium manganese iron phosphate (LMFP) product, containing 45% of the first grade, is mixed with a third washing solution including deionized water. After a third washing under ultrasonic and stirring conditions, the mixture is transferred to a fourth horizontal spiral sedimentation centrifuge for a third solid-liquid separation, yielding a third mother liquor and LFP with a solid content of 45%. The median particle size of the primary particles in the LFP is 150 nm. The conductivity of the third mother liquor is 350 μS / cm, and the sulfur content of the LFP is 0.25%. The third mother liquor is then introduced into the second washing solution for recycling during the second washing process. The second mother liquor is also introduced into the first washing solution for recycling during the first washing process. After mixing the first mother liquor and the separated mother liquor, a rotary ceramic membrane separator is used to remove any remaining trace amounts of LFP from both the first mother liquor and the separated mother liquor, resulting in a first recovered liquid phase and a small amount of LFP. The remaining trace amount of LFP is then transferred to a secondary washing tank for recycling during the second washing process.

[0154] (4) The lithium manganese iron phosphate with a solid content of 45% obtained after washing was mixed with carbon source (glucose and graphene) and deoxygenated deionized water, and ultrasonically dispersed to obtain a slurry with a solid content of 0.35%, wherein the mass ratio of glucose to graphene was 99:1, and the mass ratio of lithium manganese iron phosphate to carbon source was 0.80:0.20; the slurry was ground and refined using a nano-grinding mill to obtain a crystal slurry with a median particle size D50 of 160nm and D97 = 200nm; the crystal slurry was granulated by a high-speed centrifugal spray dryer, and the inlet air temperature was set to 250℃ and the outlet air temperature was set to 105℃ to obtain secondary lithium manganese iron phosphate particles with a median particle size of 16.5μm and a residual water content of <1%;

[0155] (5) The above-mentioned secondary particles of lithium manganese iron phosphate were sintered in an atmosphere sintering rotary furnace. The sintering temperature was set at 700℃, the heating rate was 3℃ / min, and the constant temperature sintering time was 4h. The sintered product was obtained. The sintered product was crushed by ceramic flat airflow pulverizer to obtain carbon-coated lithium manganese iron phosphate positive electrode active material with median particle size D50 of 150nm and D97 of 450nm.

[0156] (6) Under nitrogen protection, barium hydroxide octahydrate and deionized water are mixed in the fourth ultrasonic dissolving tank and stirred to dissolve at a temperature of 90°C. The dissolved liquid is then filtered through a ceramic precision filter to remove insoluble impurities, resulting in a barium hydroxide solution with a mass percentage of 40 wt%. The first recovered liquid phase obtained in step (3) is further treated with ion exchange resin to remove residual iron and manganese ions. The first recovered liquid phase with removed iron and manganese ions is mixed with the barium hydroxide solution, maintaining a molar ratio of barium ions to sulfate ions of 1:1. The reaction is carried out at 30°C to obtain a reaction solution. The reaction solution is then treated with a diaphragm filter press for second liquid phase recovery to obtain barium sulfate precipitate and second recovered liquid phase. The barium sulfate precipitate is dried and crushed by a flash dryer and a flat air jet mill to obtain a barium sulfate product with a median particle size D50 of 0.71 μm, a purity of 99.5%, and a whiteness of 98.0%.

[0157] (7) The second recovered liquid phase was concentrated using a vacuum flash evaporator. The concentration of lithium hydroxide in the concentrated second recovered liquid phase was 90 g / L. The vacuum flash evaporation temperature was set to 60°C. After drying, 958.0 kg of recovered lithium hydroxide monohydrate was obtained. The recovered lithium hydroxide monohydrate was used as the lithium source in step (1). The recycling rate of lithium hydroxide monohydrate was 99.0%.

[0158] Example 8

[0159] (1) 1451.5 kg of lithium hydroxide monohydrate, 9600 L of deoxygenated deionized water and diethylene glycol (volume ratio 2.2:1) were added to the first ultrasonic dissolving container under nitrogen protection and stirred for 30 min at a dissolving temperature of 35°C. The dissolved solution was then passed through a ceramic precision filter to remove insoluble impurities, yielding a lithium hydroxide solution. 960.8 kg of ferrous sulfate heptahydrate, 1363.2 kg of manganese sulfate monohydrate, 50 g of ascorbic acid, and 3... 000L of deoxygenated deionized water was added to the second ultrasonic dissolving tank under nitrogen protection and stirred for 30 minutes at a dissolving temperature of 35℃. The dissolved solution was then passed through a ceramic precision filter to remove insoluble impurities, yielding a mixed solution of ferrous sulfate heptahydrate and manganese sulfate monohydrate. 788.4L of 85% thermal phosphoric acid and 6600L of diethylene glycol were added to the third ultrasonic dissolving tank and mixed evenly. The mixed solution was then passed through a ceramic precision filter to remove insoluble impurities, yielding a phosphoric acid solution.

[0160] (2) Lithium hydroxide solution and phosphoric acid solution are mixed in a first SK-type tubular static mixer to obtain lithium phosphate slurry. Then, the lithium phosphate slurry is further mixed with a mixed solution of ferrous sulfate heptahydrate and manganese sulfate monohydrate in a second SK-type tubular static mixer to obtain a raw material solution. The ratio of the molar amounts of lithium ions, iron ions, manganese ions, and phosphate ions in the raw material solution is maintained at 3:1:1. The raw material solution is then transported to an ultrasonic mixing solvothermal reaction crystallization device, sealed, and the atmosphere inside the device is replaced with nitrogen to replace the air. The reaction is carried out under ultrasonic action using inductive heating (ultrasonic power is 5KW / m). 3 The heating rate was set at 3℃ / min, the reaction temperature at 180℃, and the pressure at 1.0MPa. The reaction was stopped after 3 hours of constant temperature reaction. The solid content of lithium manganese iron phosphate in the reaction system was 80.6 kg / m³. 3 ;

[0161] (3) After the above reaction is completed, the temperature of the reaction liquid is reduced to below 60°C using a spiral tube heat exchanger. The cooled reaction liquid is then transported to a first horizontal spiral sedimentation centrifuge for solid-liquid separation to obtain a mother liquor and a crude lithium manganese iron phosphate product with a solid content of 45%. The crude lithium manganese iron phosphate product with a solid content of 45% is then transported to a primary washing tank and mixed with a first washing liquid including deionized water. After a first washing under ultrasonic and stirring action, it is transported to a second horizontal spiral sedimentation centrifuge for the first solid-liquid separation to obtain a first mother liquor and a first crude lithium manganese iron phosphate product with a solid content of 45%. The first crude lithium manganese iron phosphate product with a solid content of 45% is then mixed with a second washing liquid including deionized water. After a second washing under ultrasonic and stirring action, it is transported to a third horizontal spiral sedimentation centrifuge for the second solid-liquid separation to obtain a second mother liquor and a second crude lithium manganese iron phosphate product with a solid content of 45%. The crude lithium manganese iron phosphate (LMFP) product, containing 45% of the first grade, is mixed with a third washing solution including deionized water. After a third washing under ultrasonic and stirring conditions, the mixture is transferred to a fourth horizontal spiral sedimentation centrifuge for a third solid-liquid separation, yielding a third mother liquor and LFP with a solid content of 45%. The median particle size of the primary particles in the LFP is 150 nm. The conductivity of the third mother liquor is 350 μS / cm, and the sulfur content of the LFP is 0.25%. The third mother liquor is then introduced into the second washing solution for recycling during the second washing process. The second mother liquor is also introduced into the first washing solution for recycling during the first washing process. After mixing the first mother liquor and the separated mother liquor, a rotary ceramic membrane separator is used to remove any remaining trace amounts of LFP from both the first mother liquor and the separated mother liquor, resulting in a first recovered liquid phase and a small amount of LFP. The remaining trace amount of LFP is then transferred to a secondary washing tank for recycling during the second washing process.

[0162] (4) The lithium manganese iron phosphate with a solid content of 45% obtained after washing was mixed with carbon source (glucose and graphene) and deoxygenated deionized water, and ultrasonically dispersed to obtain a slurry with a solid content of 0.35%, wherein the mass ratio of glucose to graphene was 99:1, and the mass ratio of lithium manganese iron phosphate to carbon source was 0.88:0.12; the slurry was ground and refined using a nano-grinding mill to obtain a crystal slurry with a median particle size D50 of 250nm and D97 of 350nm; the crystal slurry was granulated by a high-speed centrifugal spray dryer, with the inlet air temperature set at 250℃ and the outlet air temperature at 105℃, to obtain secondary lithium manganese iron phosphate particles with a median particle size of 13μm and a residual water content of <1%;

[0163] (5) The above-mentioned secondary particles of lithium manganese iron phosphate were sintered in an atmosphere sintering rotary furnace. The sintering temperature was set at 700℃, the heating rate was 3℃ / min, and the constant temperature sintering time was 4h. The sintered product was obtained. The sintered product was crushed by ceramic flat airflow pulverizer to obtain carbon-coated lithium manganese iron phosphate positive electrode active material with median particle size D50 of 255nm and D97 of 600nm.

[0164] (6) Under nitrogen protection, barium hydroxide octahydrate and deionized water are mixed in the fourth ultrasonic dissolving tank and stirred to dissolve at a temperature of 90°C. The dissolved liquid is then filtered through a ceramic precision filter to remove insoluble impurities, resulting in a barium hydroxide solution with a mass percentage of 40 wt%. The first recovered liquid phase obtained in step (3) is further treated with ion exchange resin to remove residual iron and manganese ions. The first recovered liquid phase with removed iron and manganese ions is mixed with the barium hydroxide solution, maintaining a molar ratio of barium ions to sulfate ions of 1:1. The reaction is carried out at 30°C to obtain a reaction solution. The reaction solution is then treated with a diaphragm filter press for second liquid phase recovery to obtain barium sulfate precipitate and second recovered liquid phase. The barium sulfate precipitate is dried and crushed by a flash dryer and a flat air jet mill to obtain a barium sulfate product with a median particle size D50 of 0.65 μm, a purity of 99.3%, and a whiteness of 98.9%.

[0165] (7) The second recovered liquid phase was concentrated using a vacuum flash evaporator. The concentration of lithium hydroxide in the concentrated second recovered liquid phase was 90 g / L. The vacuum flash evaporation temperature was set to 60 °C. After drying, 957.0 kg of recovered lithium hydroxide monohydrate was obtained. The recovered lithium hydroxide monohydrate was used as the lithium source in step (1). The recycling rate of lithium hydroxide monohydrate was 98.9%.

[0166] Example 9

[0167] (1) 1451.5 kg of lithium hydroxide monohydrate, 9600 L of deoxygenated deionized water and diethylene glycol (volume ratio 2.2:1) were added to the first ultrasonic dissolving container under nitrogen protection and stirred for 30 min at a dissolving temperature of 35°C. The dissolved solution was then passed through a ceramic precision filter to remove insoluble impurities, yielding a lithium hydroxide solution. 960.8 kg of ferrous sulfate heptahydrate, 1363.2 kg of manganese sulfate monohydrate, 50 g of ascorbic acid, and 3... 000L of deoxygenated deionized water was added to the second ultrasonic dissolving tank under nitrogen protection and stirred for 30 minutes at a dissolving temperature of 35℃. The dissolved solution was then passed through a ceramic precision filter to remove insoluble impurities, yielding a mixed solution of ferrous sulfate heptahydrate and manganese sulfate monohydrate. 788.4L of 85% thermal phosphoric acid and 6600L of diethylene glycol were added to the third ultrasonic dissolving tank and mixed evenly. The mixed solution was then passed through a ceramic precision filter to remove insoluble impurities, yielding a phosphoric acid solution.

[0168] (2) Lithium hydroxide solution and phosphoric acid solution are mixed in a first SK-type tubular static mixer to obtain lithium phosphate slurry. Then, the lithium phosphate slurry is further mixed with a mixed solution of ferrous sulfate heptahydrate and manganese sulfate monohydrate in a second SK-type tubular static mixer to obtain a raw material solution. The ratio of the molar amounts of lithium ions, iron ions, manganese ions, and phosphate ions in the raw material solution is maintained at 3:1:1. The raw material solution is then transported to an ultrasonic mixing solvothermal reaction crystallization device, sealed, and the atmosphere inside the device is replaced with nitrogen to replace the air. The reaction is carried out under ultrasonic action using inductive heating (ultrasonic power is 5KW / m). 3 The heating rate was set at 3℃ / min, the reaction temperature at 180℃, and the pressure at 1.0MPa. The reaction was stopped after 3 hours of constant temperature reaction. The solid content of lithium manganese iron phosphate in the reaction system was 80.6 kg / m³. 3 ;

[0169] (3) After the above reaction is completed, the temperature of the reaction liquid is reduced to below 60°C using a spiral tube heat exchanger. The cooled reaction liquid is then transported to a first horizontal spiral sedimentation centrifuge for solid-liquid separation to obtain a mother liquor and a crude lithium manganese iron phosphate product with a solid content of 45%. The crude lithium manganese iron phosphate product with a solid content of 45% is then transported to a primary washing tank and mixed with a first washing liquid including deionized water. After a first washing under ultrasonic and stirring action, it is transported to a second horizontal spiral sedimentation centrifuge for the first solid-liquid separation to obtain a first mother liquor and a first crude lithium manganese iron phosphate product with a solid content of 45%. The first crude lithium manganese iron phosphate product with a solid content of 45% is then mixed with a second washing liquid including deionized water. After a second washing under ultrasonic and stirring action, it is transported to a third horizontal spiral sedimentation centrifuge for the second solid-liquid separation to obtain a second mother liquor and a second crude lithium manganese iron phosphate product with a solid content of 45%. The crude lithium manganese iron phosphate (LMFP) product, containing 45% of the first grade, is mixed with a third washing solution including deionized water. After a third washing under ultrasonic and stirring conditions, the mixture is transferred to a fourth horizontal spiral sedimentation centrifuge for a third solid-liquid separation, yielding a third mother liquor and LFP with a solid content of 45%. The median particle size of the primary particles in the LFP is 150 nm. The conductivity of the third mother liquor is 350 μS / cm, and the sulfur content of the LFP is 0.25%. The third mother liquor is then introduced into the second washing solution for recycling during the second washing process. The second mother liquor is also introduced into the first washing solution for recycling during the first washing process. After mixing the first mother liquor and the separated mother liquor, a rotary ceramic membrane separator is used to remove any remaining trace amounts of LFP from both the first mother liquor and the separated mother liquor, resulting in a first recovered liquid phase and a small amount of LFP. The remaining trace amount of LFP is then transferred to a secondary washing tank for recycling during the second washing process.

[0170] (4) The lithium manganese iron phosphate with a solid content of 45% obtained after washing was mixed with carbon source (glucose and graphene) and deoxygenated deionized water, and ultrasonically dispersed to obtain a slurry with a solid content of 0.35%, wherein the mass ratio of glucose to graphene was 99:1, and the mass ratio of lithium manganese iron phosphate to carbon source was 0.88:0.12; the slurry was ground and refined using a nano-grinding mill to obtain a crystal slurry with a median particle size D50 of 100nm and D97 of 200nm; the crystal slurry was granulated by a high-speed centrifugal spray dryer, with the inlet air temperature set at 250℃ and the outlet air temperature at 105℃, to obtain secondary lithium manganese iron phosphate particles with a median particle size of 22μm and a residual water content of <1%;

[0171] (5) The above-mentioned secondary particles of lithium manganese iron phosphate were sintered in an atmosphere sintering rotary furnace. The sintering temperature was set at 700℃, the heating rate was 3℃ / min, and the constant temperature sintering time was 4h. The sintered product was obtained. The sintered product was crushed by ceramic flat airflow pulverizer to obtain carbon-coated lithium manganese iron phosphate positive electrode active material with median particle size D50 of 150nm and D97 of 450nm.

[0172] (6) Under nitrogen protection, barium hydroxide octahydrate and deionized water are mixed in the fourth ultrasonic dissolving tank and stirred to dissolve at a temperature of 90°C. The dissolved liquid is then filtered through a ceramic precision filter to remove insoluble impurities, resulting in a barium hydroxide solution with a mass percentage of 40 wt%. The first recovered liquid phase obtained in step (3) is further treated with ion exchange resin to remove residual iron and manganese ions. The first recovered liquid phase with removed iron and manganese ions is mixed with the barium hydroxide solution, maintaining a molar ratio of barium ions to sulfate ions of 1:1. The reaction is carried out at 30°C to obtain a reaction solution. The reaction solution is then treated with a diaphragm filter press for second liquid phase recovery to obtain barium sulfate precipitate and second recovered liquid phase. The barium sulfate precipitate is dried and crushed by a flash dryer and a flat air jet mill to obtain a barium sulfate product with a median particle size D50 of 0.75 μm, a purity of 99.5%, and a whiteness of 98.9%.

[0173] (7) The second recovered liquid phase was concentrated using a vacuum flash evaporator. The concentration of lithium hydroxide in the concentrated second recovered liquid phase was 90 g / L. The vacuum flash evaporation temperature was set to 70 °C. After drying, 960.9 kg of recovered lithium hydroxide monohydrate was obtained. The recovered lithium hydroxide monohydrate was used as the lithium source in step (1). The recycling rate of lithium hydroxide monohydrate was 99.3%.

[0174] Example 10

[0175] (1) 1451.5 kg of lithium hydroxide monohydrate, 9600 L of deoxygenated deionized water and diethylene glycol (volume ratio 2.2:1) were added to the first ultrasonic dissolving container under nitrogen protection and stirred for 30 min at a dissolving temperature of 35°C. The dissolved solution was then passed through a ceramic precision filter to remove insoluble impurities, yielding a lithium hydroxide solution. 960.8 kg of ferrous sulfate heptahydrate, 1363.2 kg of manganese sulfate monohydrate, 50 g of ascorbic acid, and 3... 000L of deoxygenated deionized water was added to the second ultrasonic dissolving tank under nitrogen protection and stirred for 30 minutes at a dissolving temperature of 35℃. The dissolved solution was then passed through a ceramic precision filter to remove insoluble impurities, yielding a mixed solution of ferrous sulfate heptahydrate and manganese sulfate monohydrate. 788.4L of 85% thermal phosphoric acid and 6600L of diethylene glycol were added to the third ultrasonic dissolving tank and mixed evenly. The mixed solution was then passed through a ceramic precision filter to remove insoluble impurities, yielding a phosphoric acid solution.

[0176] (2) Lithium hydroxide solution and phosphoric acid solution are mixed in a first SK type tubular static mixer to obtain lithium phosphate slurry. Then, the lithium phosphate slurry is further mixed with a mixed solution of ferrous sulfate heptahydrate and manganese sulfate monohydrate in a second SK type tubular static mixer to obtain raw material solution. The ratio of the molar amounts of lithium ions, iron ions, manganese ions, and phosphate ions in the raw material solution is maintained at 3:1:1. The raw material solution is then transported to an ultrasonic mixing solvothermal reaction crystallization device, sealed, and the atmosphere inside the device is replaced with nitrogen to replace the air. The reaction is carried out under ultrasonic action using inductive heating (ultrasonic power is 5kW / m). 3 The heating rate was set at 3℃ / min, the reaction temperature at 180℃, and the pressure at 1.0MPa. The reaction was stopped after 3 hours of constant temperature reaction. The solid content of lithium manganese iron phosphate in the reaction system was 80.6 kg / m³. 3 ;

[0177] (3) After the above reaction is completed, the temperature of the reaction liquid is reduced to below 60°C using a spiral tube heat exchanger. The cooled reaction liquid is then transported to a first horizontal spiral sedimentation centrifuge for solid-liquid separation to obtain a mother liquor and a crude lithium manganese iron phosphate product with a solid content of 45%. The crude lithium manganese iron phosphate product with a solid content of 45% is then transported to a primary washing tank and mixed with a first washing liquid including deionized water. After a first washing under ultrasonic and stirring action, it is transported to a second horizontal spiral sedimentation centrifuge for the first solid-liquid separation to obtain a first mother liquor and a first crude lithium manganese iron phosphate product with a solid content of 45%. The first crude lithium manganese iron phosphate product with a solid content of 45% is then mixed with a second washing liquid including deionized water. After a second washing under ultrasonic and stirring action, it is transported to a third horizontal spiral sedimentation centrifuge for the second solid-liquid separation to obtain a second mother liquor and a second crude lithium manganese iron phosphate product with a solid content of 45%. The crude lithium manganese iron phosphate (LMFP) product, containing 45% of the first grade, is mixed with a third washing solution including deionized water. After a third washing under ultrasonic and stirring conditions, the mixture is transferred to a fourth horizontal spiral sedimentation centrifuge for a third solid-liquid separation, yielding a third mother liquor and LFP with a solid content of 45%. The median particle size of the primary particles in the LFP is 150 nm. The conductivity of the third mother liquor is 350 μS / cm, and the sulfur content of the LFP is 0.25%. The third mother liquor is then introduced into the second washing solution for recycling during the second washing process. The second mother liquor is also introduced into the first washing solution for recycling during the first washing process. After mixing the first mother liquor and the separated mother liquor, a rotary ceramic membrane separator is used to remove any remaining trace amounts of LFP from both the first mother liquor and the separated mother liquor, resulting in a first recovered liquid phase and a small amount of LFP. The remaining trace amount of LFP is then transferred to a secondary washing tank for recycling during the second washing process.

[0178] (4) The lithium manganese iron phosphate with a solid content of 45% obtained after washing was mixed with carbon source (glucose and graphene) and deoxygenated deionized water, and ultrasonically dispersed to obtain a slurry with a solid content of 0.35%, wherein the mass ratio of glucose to graphene was 99:1, and the mass ratio of lithium manganese iron phosphate to carbon source was 0.88:0.12; the slurry was ground and refined using a nano-grinding mill to obtain a crystal slurry with a median particle size D50 of 100nm and D97 of 200nm; the crystal slurry was granulated by a high-speed centrifugal spray dryer, and the inlet air temperature was set to 250℃ and the outlet air temperature was set to 105℃ to obtain secondary lithium manganese iron phosphate particles with a median particle size of 13.5μm and a residual water content of <1%;

[0179] (5) The above-mentioned secondary particles of lithium manganese iron phosphate were sintered in an atmosphere sintering rotary furnace. The sintering temperature was set at 600℃, the heating rate was 7℃ / min, and the constant temperature sintering time was 7h. The sintered product was obtained. The sintered product was crushed by ceramic flat airflow pulverizer to obtain carbon-coated lithium manganese iron phosphate positive electrode active material with median particle size D50 of 160nm and D97 of 450nm.

[0180] (6) Under nitrogen protection, barium hydroxide octahydrate and deionized water are mixed in the fourth ultrasonic dissolving tank and stirred to dissolve at a temperature of 90°C. The dissolved liquid is then filtered through a ceramic precision filter to remove insoluble impurities, resulting in a barium hydroxide solution with a mass percentage of 40 wt%. The first recovered liquid phase obtained in step (3) is further treated with ion exchange resin to remove residual iron and manganese ions. The first recovered liquid phase with removed iron and manganese ions is mixed with the barium hydroxide solution, maintaining a molar ratio of barium ions to sulfate ions of 1:1. The reaction is carried out at 30°C to obtain a reaction solution. The reaction solution is then treated with a diaphragm filter press for second liquid phase recovery to obtain barium sulfate precipitate and second recovered liquid phase. The barium sulfate precipitate is dried and crushed by a flash dryer and a flat air jet mill to obtain a barium sulfate product with a median particle size D50 of 0.72 μm, a purity of 99.3%, and a whiteness of 99.0%.

[0181] (7) The second recovered liquid phase was concentrated using a vacuum flash evaporator. The concentration of lithium hydroxide in the concentrated second recovered liquid phase was 90 g / L. The vacuum flash evaporation temperature was set to 65°C. After drying, 959.9 kg of recovered lithium hydroxide monohydrate was obtained. The recovered lithium hydroxide monohydrate was used as the lithium source in step (1). The recycling rate of lithium hydroxide monohydrate was 99.2%.

[0182] Comparative Example 1

[0183] The preparation method of the carbon-coated lithium manganese iron phosphate positive electrode active material in this comparative example is basically the same as that in Example 1. The difference is that in step (2), the reaction temperature is 220℃ and the pressure is 2.32MPa; in step (3), the median particle size of the primary particles included in the lithium manganese iron phosphate is 300nm.

[0184] Comparative Example 2

[0185] The preparation method of the carbon-coated lithium manganese iron phosphate positive electrode active material in this comparative example is basically the same as that in Example 1. The difference is that in step (2), there is no ultrasonic action during the reaction process; in step (3), the median particle size of the primary particles included in the lithium manganese iron phosphate is 450 nm.

[0186] Comparative Example 3

[0187] The preparation method of the carbon-coated lithium manganese iron phosphate positive electrode active material in this comparative example is basically the same as that in Example 1. The difference is that in step (1), the amount of lithium hydroxide monohydrate added is 362.88 kg, the amount of ferrous sulfate heptahydrate added is 240.24 kg, the amount of manganese sulfate monohydrate added is 340.91 kg, and the amount of 85% thermal phosphoric acid used is 192.65 L.

[0188] In step (2), there was no ultrasonic treatment during the reaction, the heating rate was 20℃ / min, the reaction temperature was 160℃, the pressure was 0.62MPa, the reaction was carried out at a constant temperature for 4 hours, and the solid content of lithium manganese iron phosphate in the reaction system was 22.67kg / m³. 3 ;

[0189] In step (3), the median particle size of the primary particles included in lithium manganese iron phosphate is 150 nm;

[0190] In step (4), the median particle size D50 of the crystal slurry is 150 nm, D97 is 450 nm, and the median particle size after granulation is 5 μm.

[0191] In step (5), the sintering temperature is 700℃, the heating rate is 5℃ / min, the isothermal sintering time is 3h, and the median particle size D50 of the carbon-coated lithium manganese iron phosphate cathode active material is 160nm and D97 is 480nm.

[0192] Comparative Example 4

[0193] The preparation method of the carbon-coated lithium manganese iron phosphate cathode active material in this comparative example is basically the same as that in Example 1, except that in step (2), the ultrasonic power is 2kW / m 3 In step (3), the median particle size of the primary particles included in lithium manganese iron phosphate is 280 nm.

[0194] Test case

[0195] 1. XRD tests were performed on the lithium manganese iron phosphate after washing in Example 1 above. The test results are shown in [Figure 1]. Figure 1 .

[0196] Figure 1 The image shows the XRD pattern of lithium manganese iron phosphate after washing in Example 1. Figure 1 As can be seen, no impurity peaks appeared in the XRD image, indicating that the lithium manganese iron phosphate prepared in Example 1 has high purity.

[0197] 2. The carbon-coated lithium manganese iron phosphate cathode active materials prepared in the above examples and comparative examples are applied to lithium-ion batteries. The specific steps are as follows:

[0198] The carbon-coated lithium manganese iron phosphate positive electrode active material (1.5000±0.0020g) prepared in the above examples and comparative examples was mixed with conductive agent carbon black SP (0.0833±0.0020g) and N-methylpyrrolidone solution of polyvinylidene fluoride (4.1650±0.0200g) at a mass ratio of 90:5:5 to obtain a positive electrode active layer slurry. The positive electrode active layer slurry was coated on the surface of a 15*30cm aluminum foil using a Kejing coating machine and a four-sided coating machine, with a coating surface density of 70g / m². 2 Approximately g / m 2 The electrodes were dried at 120℃ for more than 1 hour. One-third of the dried electrodes were then cut off, and the cut electrodes were compacted using a roller press to achieve a compaction density of 1.9 g / cm³. 3 The positive electrode is obtained; in the glove box, the negative electrode battery shell, positive electrode, electrolyte (100 μL), polyethylene separator, electrolyte (100 μL), lithium metal sheet (smooth side down), gasket, spring sheet (large opening facing the gasket), and positive electrode battery shell are assembled in sequence to obtain a button cell battery, wherein the electrolyte is Tinci STLD-3 model.

[0199] The charge-discharge performance of the coin cells prepared above was tested, and the specific steps are as follows:

[0200] First, let it stand for 3 minutes, then charge at a 0.2C rate to ≥4.35V, then charge at a constant voltage of 4.35V until the current rate is ≤0.02C, let it stand for 3 minutes, and then discharge at a 0.2C rate until the voltage is ≤2.5V. Repeat this cycle twice, recording the values ​​every 30 seconds to obtain the initial 0.2C capacity and the median 0.2C discharge voltage. Next, let it stand for 3 minutes, then charge at a 1C rate to ≥4.35V, then charge at a constant voltage of 4.35V until the current rate is ≤0.02C, let it stand for 3 minutes, and then discharge at a 1C rate until the voltage is ≤2.5V. Repeat this cycle twice, recording the values ​​every 30 seconds to obtain the initial 1C capacity and the median 1C discharge voltage.

[0201] The test results are shown in Table 1 and Figure 2 .

[0202] Figure 2 The charge-discharge curves of the coin cell with carbon-coated lithium manganese iron phosphate cathode active material in Example 1 of this invention are obtained from... Figure 2 It can be seen that the coin cell containing the carbon-coated lithium manganese iron phosphate cathode active material of Example 1 has a high discharge capacity and median discharge voltage, thus indicating that the cathode active material of the present invention has good electrochemical performance.

[0203] Table 1

[0204]

[0205]

[0206] As shown in Table 1:

[0207] Compared to Comparative Examples 1-3, the carbon-coated lithium manganese iron phosphate cathode active materials prepared in Examples 1-10 exhibit higher initial capacity and higher median discharge voltage. The highest initial capacity at 0.2C reaches 154.3 mAh / g, and the highest median discharge voltage at 0.2C reaches 4.072 V. The highest initial capacity at 1.0C reaches 150.4 mAh / g, and the highest median discharge voltage at 1.0C reaches 3.950 V. Comparative Example 4 is a carbon-coated lithium manganese iron phosphate cathode active material prepared at a lower feed solution concentration. Its electrochemical performance is similar to that of Example 1, but its solid content is 22.67 kg / m³. 3 The production efficiency is relatively low. Therefore, the preparation method of carbon-coated lithium manganese iron phosphate cathode active material of the present invention can prepare primary lithium manganese iron phosphate particles with small particle size under the condition of high solid content in the reaction system. Thus, the carbon-coated lithium manganese iron phosphate cathode active material obtained by mixing and sintering lithium manganese iron phosphate with carbon source has better electrochemical performance and can significantly improve production efficiency.

[0208] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for preparing a carbon-coated lithium manganese iron phosphate cathode active material, characterized in that, Includes the following steps: (1) Under 0.2–1.6 MPa and at 120–200 °C, a raw material solution including lithium source, iron source, manganese source and phosphorus source is subjected to ultrasonic reaction to obtain lithium manganese iron phosphate; in the ultrasonic reaction, the ultrasonic power is not less than 3 KW / m 3 The conditions for the ultrasonic reaction are: a heating rate of 0.5–20 °C / min and a holding time of 0.5–24 h. (2) The lithium manganese iron phosphate and a carbon source are sintered under a protective atmosphere to obtain the carbon-coated lithium manganese iron phosphate positive electrode active material; The solid content of lithium manganese iron phosphate in the reaction system after the ultrasonic reaction is completed is 40-100 kg / m³. 3 ; The lithium manganese iron phosphate comprises primary particles and secondary particles formed from the primary particles, wherein the median particle size of the primary particles is 50–200 nm. Step (1) further includes, after the ultrasonic reaction is completed, solid-liquid separation treatment is performed on the reaction system to obtain crude lithium manganese iron phosphate and separation mother liquor; the crude lithium manganese iron phosphate is washed to obtain lithium manganese iron phosphate and washing mother liquor. The washing process includes: The crude lithium manganese iron phosphate product is first washed with a first washing solution including deionized water to obtain a first crude lithium manganese iron phosphate product and a first mother liquor. The first crude lithium manganese iron phosphate product is washed a second time using a second washing solution including deionized water to obtain a second crude lithium manganese iron phosphate product and a second mother liquor. The crude product of the second lithium manganese iron phosphate is washed a third time using a third washing solution including deionized water to obtain lithium manganese iron phosphate and a third mother liquor. The solids in the separated mother liquor and the first mother liquor are introduced into the first crude lithium manganese iron phosphate product for recycling to participate in the second washing; and / or... The second mother liquor is introduced into the first washing solution and circulated to participate in the first washing process; and / or, The third mother liquor is introduced into the second washing liquid and circulated to participate in the second washing process.

2. The method for preparing carbon-coated lithium manganese iron phosphate cathode active material according to claim 1, characterized in that, In step (1), the ratio of the molar amount of lithium ions, the total molar amount of iron ions and manganese ions, and the molar amount of phosphate ions in the raw material solution is (2.7~3.2):1:

1.

3. The method for preparing carbon-coated lithium manganese iron phosphate cathode active material according to claim 1 or 2, characterized in that, The mass ratio of lithium manganese iron phosphate to the carbon source is 0.85:0.15 to 0.95:0.

05.

4. The method for preparing carbon-coated lithium manganese iron phosphate cathode active material according to claim 1 or 2, characterized in that, Step (2) includes mixing the lithium manganese iron phosphate, the carbon source, and deionized water to obtain a slurry; grinding the slurry to obtain a crystal slurry with a median particle size D50 of 50-200 nm and D97 < 300 nm; granulating the crystal slurry to obtain lithium manganese iron phosphate mixed carbon source particles with a median particle size D50 of 1-20 μm and a residual water content of < 1%; and sintering the lithium manganese iron phosphate mixed carbon source particles under a protective atmosphere to obtain the carbon-coated lithium manganese iron phosphate positive electrode active material.

5. The method for preparing carbon-coated lithium manganese iron phosphate cathode active material according to claim 1 or 2, characterized in that, The sintering conditions are as follows: sintering temperature is 650-750℃, heating rate is 1-5℃ / min, and holding time is 1.0-6.0h.

6. The method for preparing carbon-coated lithium manganese iron phosphate cathode active material according to claim 1, characterized in that, The lithium source is lithium hydroxide monohydrate, the iron source is ferrous sulfate heptahydrate, the manganese source is manganese sulfate monohydrate, and the phosphorus source is phosphoric acid. The separated mother liquor and the first mother liquor are subjected to a first liquid phase recovery treatment to obtain a first recovered liquid phase. The first recovered liquid phase is mixed with barium hydroxide octahydrate solution for reaction. The reaction solution is subjected to a second liquid phase recovery treatment to obtain a second recovered liquid phase. The second recovered liquid phase is concentrated and dried to obtain recovered lithium hydroxide monohydrate. The recovered lithium hydroxide is used as the lithium source. The molar ratio of barium ions in the barium hydroxide octahydrate solution to sulfate ions in the first recovered liquid phase is 1:(1-1.01), and the reaction temperature is 15-90℃.

7. A production system for carbon-coated lithium manganese iron phosphate cathode active material, characterized in that, The method for preparing carbon-coated lithium manganese iron phosphate cathode active material according to any one of claims 1-6 includes an ultrasonic mixing solvothermal reaction crystallization apparatus and a sintering apparatus. The ultrasonic mixed solvothermal reaction crystallization device is used to prepare the lithium manganese iron phosphate, and the sintering device is used to sinter the lithium manganese iron phosphate and the carbon source. The output port of the ultrasonic mixed solvothermal reaction crystallization device is connected to the input port of the sintering device.

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